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

BSD/Linux/OS390x 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

@@ -655,19 +655,24 @@ 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;
   }
 
-  // Fall back to the old expected compiler-rt name if the new one does not
-  // exist.
+#if !LLVM_ENABLE_PER_TARGET_RUNTIME_DIR
+  // When LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is off, fall back to the old
+  // expected compiler-rt name if the new one does not exist.
   CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/true);
-  SmallString<128> Path(getCompilerRTPath());
+  Path = getCompilerRTPath();
   llvm::sys::path::append(Path, CRTBasename);
+#endif
   return std::string(Path);
 }
 

clang/test/Driver/Inputs/resource_dir/lib/x86_64-unknown-linux/clang_rt.crtbegin.o

@@ -61,15 +61,15 @@
 // CHECK-LD-RT: "--eh-frame-hdr"
 // CHECK-LD-RT: "-m" "elf_x86_64"
 // CHECK-LD-RT: "-dynamic-linker"
-// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtbegin-x86_64.o"
+// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-linux{{/|\\\\}}clang_rt.crtbegin.o"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib/gcc/x86_64-unknown-linux/10.2.0"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib/gcc/x86_64-unknown-linux/10.2.0/../../../../x86_64-unknown-linux/lib"
 // CHECK-LD-RT: "-L[[SYSROOT]]/lib"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib"
-// CHECK-LD-RT: libclang_rt.builtins-x86_64.a"
+// CHECK-LD-RT: libclang_rt.builtins.a"
 // CHECK-LD-RT: "-lc"
-// CHECK-LD-RT: libclang_rt.builtins-x86_64.a"
-// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtend-x86_64.o"
+// CHECK-LD-RT: libclang_rt.builtins.a"
+// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-linux{{/|\\\\}}clang_rt.crtend.o"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=i686-unknown-linux \
@@ -84,15 +84,15 @@
 // CHECK-LD-RT-I686: "--eh-frame-hdr"
 // CHECK-LD-RT-I686: "-m" "elf_i386"
 // CHECK-LD-RT-I686: "-dynamic-linker"
-// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtbegin-i386.o"
+// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}i686-unknown-linux{{/|\\\\}}clang_rt.crtbegin.o"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib/gcc/i686-unknown-linux/10.2.0"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib/gcc/i686-unknown-linux/10.2.0/../../../../i686-unknown-linux/lib"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/lib"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib"
-// CHECK-LD-RT-I686: libclang_rt.builtins-i386.a"
+// CHECK-LD-RT-I686: libclang_rt.builtins.a"
 // CHECK-LD-RT-I686: "-lc"
-// CHECK-LD-RT-I686: libclang_rt.builtins-i386.a"
-// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtend-i386.o"
+// CHECK-LD-RT-I686: libclang_rt.builtins.a"
+// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}i686-unknown-linux{{/|\\\\}}clang_rt.crtend.o"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=arm-linux-androideabi \

clang/test/Driver/Inputs/resource_dir/lib/x86_64-unknown-linux/clang_rt.crtend.o

@@ -21,6 +21,8 @@
 /* Doesn't use `cmakedefine` because it is allowed to be empty. */
 #define LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}"
 
+#cmakedefine01 LLVM_ENABLE_PER_TARGET_RUNTIME_DIR
+
 /* Define if threads enabled */
 #cmakedefine01 LLVM_ENABLE_THREADS