[Driver] Improve error when a compiler-rt library is not found

BSD, Linux, and z/OS enable `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` by default.
When a compiler-rt library is not found, we currently report an
incorrect filename `libclang_rt.XXX-$arch.a`
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/linux/libclang_rt.asan-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 report:
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```

Link: https://discourse.llvm.org/t/runtime-directory-fallback/76860

Author: MaskRay

clang/lib/Driver/ToolChain.cpp

@@ -656,19 +656,29 @@ std::string ToolChain::getCompilerRT(const ArgList &Args, StringRef Component,
   // Check for runtime files in the new layout without the architecture first.
   std::string CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/false);
+  SmallString<128> Path;
   for (const auto &LibPath : getLibraryPaths()) {
     SmallString<128> P(LibPath);
     llvm::sys::path::append(P, CRTBasename);
     if (getVFS().exists(P))
       return std::string(P);
+    if (Path.empty())
+      Path = P;
   }
+  if (getTriple().isOSAIX())
+    Path.clear();
 
-  // Fall back to the old expected compiler-rt name if the new one does not
-  // exist.
+  // Check the filename for the old layout if the new one does not exist.
   CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/true);
-  SmallString<128> Path(getCompilerRTPath());
-  llvm::sys::path::append(Path, CRTBasename);
+  SmallString<128> OldPath(getCompilerRTPath());
+  llvm::sys::path::append(OldPath, CRTBasename);
+  if (Path.empty() || getVFS().exists(OldPath))
+    return std::string(OldPath);
+
+  // If none is found, use a file name from the new layout, which may get
+  // printed in an error message, aiding users in knowing what Clang is
+  // looking for.
   return std::string(Path);
 }