Apply the same fallbacks as runtimes search for stdlib search

[glandium]

Summary

When building clang with e.g. LLVM_ENABLE_RUNTIMES=libcxx;libunwind,
those runtimes end up in the stdlib search directory, and when
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is set, that ends up in a
target-specific subdirectory. The stdlib search does handle the
situation, but when the target in question is Android, the same issues
as those that required fallbacks for runtimes search apply.

Traditionally, those libraries are shipped as part of the Android NDK,
but when one builds their own clang for Android, they may want to use
the runtimes from the same version rather than the ones from the NDK.

[thakis]

Is it possible to test this?

[glandium]

In D146664#4352695, @thakis wrote:

Is it possible to test this?

I don't know how.

[glandium]

Superseded by D159292 + D159293.

clang/lib/Driver/ToolChain.cpp

 }
 
 const char *ToolChain::getCompilerRTArgString(const llvm::opt::ArgList &Args,
                                               StringRef Component,
                                               FileType Type) const {
   return Args.MakeArgString(getCompilerRT(Args, Component, Type));
 }
 
-ToolChain::path_list ToolChain::getRuntimePaths() const {
-  path_list Paths;
-  auto addPathForTriple = [this, &Paths](const llvm::Triple &Triple) {
-    SmallString<128> P(D.ResourceDir);
-    llvm::sys::path::append(P, "lib", Triple.str());
-    Paths.push_back(std::string(P.str()));
-  };
-
-  addPathForTriple(getTriple());
+template <typename F>
+static void fillPaths(const ToolChain &TC, F addPathForTriple) {
+  addPathForTriple(TC.getTriple());
 
   // When building with per target runtime directories, various ways of naming
   // the Arm architecture may have been normalised to simply "arm".
   // For example "armv8l" (Armv8 AArch32 little endian) is replaced with "arm".
   // Since an armv8l system can use libraries built for earlier architecture
   // versions assuming endian and float ABI match.
   //
   // Original triple: armv8l-unknown-linux-gnueabihf
   //  Runtime triple: arm-unknown-linux-gnueabihf
   //
   // We do not do this for armeb (big endian) because doing so could make us
   // select little endian libraries. In addition, all known armeb triples only
   // use the "armeb" architecture name.
   //
   // M profile Arm is bare metal and we know they will not be using the per
   // target runtime directory layout.
-  if (getTriple().getArch() == Triple::arm && !getTriple().isArmMClass()) {
-    llvm::Triple ArmTriple = getTriple();
+  if (TC.getTriple().getArch() == Triple::arm &&
+      !TC.getTriple().isArmMClass()) {
+    llvm::Triple ArmTriple = TC.getTriple();
     ArmTriple.setArch(Triple::arm);
     addPathForTriple(ArmTriple);
   }
 
   // Android targets may include an API level at the end. We still want to fall
   // back on a path without the API level.
-  if (getTriple().isAndroid() &&
-      getTriple().getEnvironmentName() != "android") {
-    llvm::Triple TripleWithoutLevel = getTriple();
+  if (TC.getTriple().isAndroid() &&
+      TC.getTriple().getEnvironmentName() != "android") {
+    llvm::Triple TripleWithoutLevel = TC.getTriple();
     TripleWithoutLevel.setEnvironmentName("android");
     addPathForTriple(TripleWithoutLevel);
   }
+}
 
+ToolChain::path_list ToolChain::getRuntimePaths() const {
+  path_list Paths;
+  auto addPathForTriple = [this, &Paths](const llvm::Triple &Triple) {
+    SmallString<128> P(D.ResourceDir);
+    llvm::sys::path::append(P, "lib", Triple.str());
+    Paths.push_back(std::string(P.str()));
+  };
+  fillPaths(*this, addPathForTriple);
   return Paths;
 }
 
 ToolChain::path_list ToolChain::getStdlibPaths() const {
   path_list Paths;
+  auto addPathForTriple = [this, &Paths](const llvm::Triple &Triple) {
     SmallString<128> P(D.Dir);
-  llvm::sys::path::append(P, "..", "lib", getTripleString());
+    llvm::sys::path::append(P, "..", "lib", Triple.str());
     Paths.push_back(std::string(P.str()));
-
+  };
+  fillPaths(*this, addPathForTriple);
   return Paths;
 }
 
 std::string ToolChain::getArchSpecificLibPath() const {
   SmallString<128> Path(getDriver().ResourceDir);
   llvm::sys::path::append(Path, "lib", getOSLibName(),
                           llvm::Triple::getArchTypeName(getArch()));
   return std::string(Path.str());