Driver: Prefer vendor supplied gcc toolchain

[tstellar]

Summary

This patch fixes an issue on Fedora where if you had the x86_64 cross
compiler installed on your x86_64 system, then clang would use that compiler
as the default toolchain. This was happening because the cross compiler
is installed to /usr/lib/gcc/x86_64-linux-gnu/ and this directory comes before
the default compiler directory (/usr/lib/gcc/x86_64-redhat-linux/) in the search
list.

This patch re-orders the search list so that vendor supplied gcc toolchains
are selected before toolchains with a generic target, which should prevent
these kind of issues on other OSes too.

[tstellar]

Ping

[bruno]

The change seems fine.

Perhaps add tests to make sure this won't cause problems in the future? Something along the lines of test/Driver/linux-ld.c?

[tstellar]

I've implemented an alternative solution here that I think is a little more robust: https://reviews.llvm.org/D52861

lib/Driver/ToolChains/Gnu.cpp

/*static*/ void Generic_GCC::GCCInstallationDetector::CollectLibDirsAndTriples(
     SmallVectorImpl<StringRef> &TripleAliases,
     SmallVectorImpl<StringRef> &BiarchLibDirs,
     SmallVectorImpl<StringRef> &BiarchTripleAliases) {
   // Declare a bunch of static data sets that we'll select between below. These
   // are specifically designed to always refer to string literals to avoid any
   // lifetime or initialization issues.
   static const char *const AArch64LibDirs[] = {"/lib64", "/lib"};
   static const char *const AArch64Triples[] = {
-      "aarch64-none-linux-gnu", "aarch64-linux-gnu", "aarch64-linux-android",
-      "aarch64-redhat-linux", "aarch64-suse-linux"};
+      "aarch64-redhat-linux", "aarch64-suse-linux",
+      "aarch64-none-linux-gnu", "aarch64-linux-gnu", "aarch64-linux-android"};
   static const char *const AArch64beLibDirs[] = {"/lib"};
   static const char *const AArch64beTriples[] = {"aarch64_be-none-linux-gnu",
                                                  "aarch64_be-linux-gnu"};
 
   static const char *const ARMLibDirs[] = {"/lib"};
   static const char *const ARMTriples[] = {"arm-linux-gnueabi",
                                            "arm-linux-androideabi"};
-  static const char *const ARMHFTriples[] = {"arm-linux-gnueabihf",
-                                             "armv7hl-redhat-linux-gnueabi",
+  static const char *const ARMHFTriples[] = {"armv7hl-redhat-linux-gnueabi",
                                              "armv6hl-suse-linux-gnueabi",
-                                             "armv7hl-suse-linux-gnueabi"};
+                                             "armv7hl-suse-linux-gnueabi",
+                                             "arm-linux-gnueabihf",
+  };
   static const char *const ARMebLibDirs[] = {"/lib"};
   static const char *const ARMebTriples[] = {"armeb-linux-gnueabi",
                                              "armeb-linux-androideabi"};
   static const char *const ARMebHFTriples[] = {
       "armeb-linux-gnueabihf", "armebv7hl-redhat-linux-gnueabi"};
 
   static const char *const X86_64LibDirs[] = {"/lib64", "/lib"};
   static const char *const X86_64Triples[] = {
-      "x86_64-linux-gnu",       "x86_64-unknown-linux-gnu",
-      "x86_64-pc-linux-gnu",    "x86_64-redhat-linux6E",
-      "x86_64-redhat-linux",    "x86_64-suse-linux",
-      "x86_64-manbo-linux-gnu", "x86_64-linux-gnu",
-      "x86_64-slackware-linux", "x86_64-linux-android",
+      "x86_64-redhat-linux6E",    "x86_64-redhat-linux",
+      "x86_64-suse-linux",        "x86_64-slackware-linux",
+      "x86_64-manbo-linux-gnu",   "x86_64-linux-gnu",
+      "x86_64-unknown-linux-gnu", "x86_64-pc-linux-gnu",
+      "x86_64-linux-gnu",         "x86_64-linux-android",
       "x86_64-unknown-linux"};
   static const char *const X32LibDirs[] = {"/libx32"};
   static const char *const X86LibDirs[] = {"/lib32", "/lib"};
   static const char *const X86Triples[] = {
-      "i686-linux-gnu",       "i686-pc-linux-gnu",     "i486-linux-gnu",
-      "i386-linux-gnu",       "i386-redhat-linux6E",   "i686-redhat-linux",
-      "i586-redhat-linux",    "i386-redhat-linux",     "i586-suse-linux",
-      "i486-slackware-linux", "i686-montavista-linux", "i686-linux-android",
+      "i386-redhat-linux6E",   "i686-redhat-linux", "i586-redhat-linuxll",
+      "i386-redhat-linux",     "i586-suse-linux",   "i486-slackware-linux",
+      "i686-montavista-linux", "i686-linux-gnu",    "i686-pc-linux-gnu",
+      "i486-linux-gnu",        "i386-linux-gnu",    "i686-linux-android",
       "i586-linux-gnu"};
 
   static const char *const MIPSLibDirs[] = {"/lib"};
   static const char *const MIPSTriples[] = {"mips-linux-gnu", "mips-mti-linux",
                                             "mips-mti-linux-gnu",
                                             "mips-img-linux-gnu"};
   static const char *const MIPSELLibDirs[] = {"/lib"};
   static const char *const MIPSELTriples[] = {"mipsel-linux-gnu",
/*static*/ void Generic_GCC::GCCInstallationDetector::CollectLibDirsAndTriples(
   static const char *const MIPSELAndroidTriples[] = {"mipsel-linux-android"};
   static const char *const MIPS64ELAndroidLibDirs[] = {"/lib64", "/lib",
                                                        "/libr2", "/libr6"};
   static const char *const MIPS64ELAndroidTriples[] = {
       "mips64el-linux-android"};
 
   static const char *const PPCLibDirs[] = {"/lib32", "/lib"};
   static const char *const PPCTriples[] = {
-      "powerpc-linux-gnu", "powerpc-unknown-linux-gnu", "powerpc-linux-gnuspe",
-      "powerpc-suse-linux", "powerpc-montavista-linuxspe"};
+      "powerpc-suse-linux", "powerpc-montavista-linuxspe",
+      "powerpc-linux-gnu", "powerpc-unknown-linux-gnu", "powerpc-linux-gnuspe"};
   static const char *const PPC64LibDirs[] = {"/lib64", "/lib"};
   static const char *const PPC64Triples[] = {
-      "powerpc64-linux-gnu", "powerpc64-unknown-linux-gnu",
-      "powerpc64-suse-linux", "ppc64-redhat-linux"};
+      "powerpc64-suse-linux", "ppc64-redhat-linux",
+      "powerpc64-linux-gnu", "powerpc64-unknown-linux-gnu"};
   static const char *const PPC64LELibDirs[] = {"/lib64", "/lib"};
   static const char *const PPC64LETriples[] = {
-      "powerpc64le-linux-gnu", "powerpc64le-unknown-linux-gnu",
-      "powerpc64le-suse-linux", "ppc64le-redhat-linux"};
+      "powerpc64le-suse-linux", "ppc64le-redhat-linux",
+      "powerpc64le-linux-gnu", "powerpc64le-unknown-linux-gnu"};
 
   static const char *const RISCV32LibDirs[] = {"/lib", "/lib32"};
   static const char *const RISCVTriples[] = {"riscv32-unknown-linux-gnu",
                                              "riscv64-unknown-linux-gnu"};
 
   static const char *const SPARCv8LibDirs[] = {"/lib32", "/lib"};
   static const char *const SPARCv8Triples[] = {"sparc-linux-gnu",
                                                "sparcv8-linux-gnu"};
   static const char *const SPARCv9LibDirs[] = {"/lib64", "/lib"};
   static const char *const SPARCv9Triples[] = {"sparc64-linux-gnu",
                                                "sparcv9-linux-gnu"};
 
   static const char *const SystemZLibDirs[] = {"/lib64", "/lib"};
   static const char *const SystemZTriples[] = {
-      "s390x-linux-gnu", "s390x-unknown-linux-gnu", "s390x-ibm-linux-gnu",
-      "s390x-suse-linux", "s390x-redhat-linux"};
+      "s390x-ibm-linux-gnu", "s390x-suse-linux", "s390x-redhat-linux",
+      "s390x-linux-gnu", "s390x-unknown-linux-gnu"};
 
 
   using std::begin;
   using std::end;
 
   if (TargetTriple.getOS() == llvm::Triple::Solaris) {
     static const char *const SolarisLibDirs[] = {"/lib"};
     static const char *const SolarisSparcV8Triples[] = {