[Driver] Ensure ToolChain::LibraryPaths is not empty for non-Darwin

Follow-up to #81037.

ToolChain::LibraryPaths holds the new compiler-rt library directory
(e.g. `/tmp/Debug/lib/clang/19/lib/x86_64-unknown-linux-gnu`). However,
it might be empty when the directory does not exist (due to the `if
(getVFS().exists(P))` change in https://reviews.llvm.org/D158475).

If neither the old/new compiler-rt library directories exists, we would
suggest the undesired old compiler-rt file name:

```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=memory -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/linux/libclang_rt.msan-x86_64.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```

With this change, we will correctly suggest the new compiler-rt file name.

Fix #87150

Pull Request: https://github.com/llvm/llvm-project/pull/87866

1 files changed, 7 insertions, 1 deletions

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 03450fc..237092e 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -796,7 +796,13 @@ ToolChain::getTargetSubDirPath(StringRef BaseDir) const {
 std::optional<std::string> ToolChain::getRuntimePath() const {
   SmallString<128> P(D.ResourceDir);
   llvm::sys::path::append(P, "lib");
-  return getTargetSubDirPath(P);
+  if (auto Ret = getTargetSubDirPath(P))
+    return Ret;
+  // Darwin does not use per-target runtime directory.
+  if (Triple.isOSDarwin())
+    return {};
+  llvm::sys::path::append(P, Triple.str());
+  return std::string(P);
 }
 
 std::optional<std::string> ToolChain::getStdlibPath() const {