[Driver] Search the toolchain dir with -print-file-name

[phosek]

Summary

This is useful when looking for directories or files relative to the
toolchain root, e.g. include/c++/v1. This change also adds a test
to make sure this functionality doesn't regress in the future.

[mcgrathr]

lgtm

clang/lib/Driver/Driver.cpp

for (const auto &Dir : P) {
         return P.str().str();
     }
     return None;
   };
 
   if (auto P = SearchPaths(PrefixDirs))
     return *P;
 
+  SmallString<128> D(Dir);
+  llvm::sys::path::append(D, "..", Name);
+  if (llvm::sys::fs::exists(Twine(D)))
+    return D.str();
+
   SmallString<128> R(ResourceDir);
   llvm::sys::path::append(R, Name);
   if (llvm::sys::fs::exists(Twine(R)))
     return R.str();
 
   SmallString<128> P(TC.getCompilerRTPath());
   llvm::sys::path::append(P, Name);
   if (llvm::sys::fs::exists(Twine(P)))

clang/test/Driver/print-file-name.c

+// Test that -print-file-name finds the correct file.
+
+// RUN: %clang -print-file-name=share/asan_blacklist.txt 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --target=x86_64-linux-gnu \
+// RUN:   | FileCheck --check-prefix=CHECK-RESOURCE-DIR %s
+// CHECK-RESOURCE-DIR: resource_dir{{/|\\\\}}share{{/|\\\\}}asan_blacklist.txt
+
+// RUN: %clang -print-file-name=libclang_rt.builtins.a 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     --target=x86_64-linux-gnu \
+// RUN:   | FileCheck --check-prefix=CHECK-COMPILER-RT %s
+// CHECK-COMPILER-RT: resource_dir_with_per_target_subdir{{/|\\\\}}lib{{/|\\\\}}x86_64-linux-gnu{{/|\\\\}}libclang_rt.builtins.a
+
+// RUN: %clang -print-file-name=include/c++/v1 2>&1 \
+// RUN:     -ccc-install-dir %S/Inputs/basic_linux_libcxx_tree/usr/bin \
+// RUN:     --target=x86_64-linux-gnu \
+// RUN:   | FileCheck --check-prefix=CHECK-INSTALL-DIR %s
+// CHECK-INSTALL-DIR: basic_linux_libcxx_tree{{/|\\\\}}usr{{/|\\\\}}bin{{/|\\\\}}..{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1