Driver: Add a gcc equivalent triple to the list of triples to search

[tstellar]

Summary

There are some gcc triples, like x86_64-redhat-linux, that provide the
same behavior as a clang triple with a similar name (e.g.
x86_64-redhat-linux-gnu). When searching for a gcc install, also search
for a gcc equivalent triple if one exists.

[MaskRay]

I think we should reduce normalization for include/library path detection in the clang driver (D110663), instead of adding more customization for each platform.
Some clang driver code uses os and environment to check feature compatibility with a target triple. Due to the limitation of the constructor llvm::Triple::Triple (os/environ is wrong if a un-normalized triple string is passed), such code can keep using the normalized triple.

clang/lib/Driver/ToolChains/Gnu.cpp

static llvm::StringRef getGCCToolchainDir(const ArgList &Args,
   // GCC_INSTALL_PREFIX specifies the gcc installation for the default
   // sysroot and is likely not valid with a different sysroot.
   if (!SysRoot.empty())
     return "";
 
   return GCC_INSTALL_PREFIX;
 }
 
+/// This function takes a 'clang' triple and converts it to an equivalent gcc
+/// triple.
+static const char *ConvertToGccTriple(StringRef CandidateTriple) {
+  return llvm::StringSwitch<const char *>(CandidateTriple)
+      .Case("aarch64-redhat-linux-gnu", "aarch64-redhat-linux")
+      .Case("i686-redhat-linux-gnu", "i686-redhat-linux")
+      .Case("ppc64le-redhat-linux-gnu", "ppc64le-redhat-linux")
+      .Case("s390x-redhat-linux-gnu", "s390x-redhat-linux")
+      .Case("x86_64-redhat-linux-gnu", "x86_64-redhat-linux")
+      .Default(NULL);
+}
+
 /// Initialize a GCCInstallationDetector from the driver.
 ///
 /// This performs all of the autodetection and sets up the various paths.
 /// Once constructed, a GCCInstallationDetector is essentially immutable.
 ///
 /// FIXME: We shouldn't need an explicit TargetTriple parameter here, and
 /// should instead pull the target out of the driver. This is currently
 /// necessary because the driver doesn't store the final version of the target
 /// triple.
 void Generic_GCC::GCCInstallationDetector::init(
     const llvm::Triple &TargetTriple, const ArgList &Args,
     ArrayRef<std::string> ExtraTripleAliases) {
   llvm::Triple BiarchVariantTriple = TargetTriple.isArch32Bit()
                                          ? TargetTriple.get64BitArchVariant()
                                          : TargetTriple.get32BitArchVariant();
   // The library directories which may contain GCC installations.
   SmallVector<StringRef, 4> CandidateLibDirs, CandidateBiarchLibDirs;
   // The compatible GCC triples for this particular architecture.
   SmallVector<StringRef, 16> CandidateTripleAliases;
   SmallVector<StringRef, 16> CandidateBiarchTripleAliases;
+
+  // In some cases gcc uses a slightly different triple than clang for the
+  // same target.  Convert the clang triple to the gcc equivalent and use that
+  // to search for the gcc install.
+  const char *ConvertedTriple = ConvertToGccTriple(TargetTriple.str());
+  if (ConvertedTriple) {
+    CandidateTripleAliases.push_back(ConvertedTriple);
+    CandidateBiarchTripleAliases.push_back(ConvertedTriple);
+  }
+
   CollectLibDirsAndTriples(TargetTriple, BiarchVariantTriple, CandidateLibDirs,
                            CandidateTripleAliases, CandidateBiarchLibDirs,
                            CandidateBiarchTripleAliases);
 
   // Compute the set of prefixes for our search.
   SmallVector<std::string, 8> Prefixes;
   StringRef GCCToolchainDir = getGCCToolchainDir(Args, D.SysRoot);
   if (GCCToolchainDir != "") {