[clang] [Driver] Add clang's relative `../lib` path only when in build tree

[MaskRay]

Summary

Change the code responsible for adding ../lib directory relative
to the clang executable directory to apply only if clang is actually
run from the build directory. According to the existing comment, this
is what the purpose of the code is.

Before, the addition was unconditional and therefore it was added
for installed clang as well. Unfortunately, this meant that on 64-bit
Gentoo systems the effective /usr/lib/llvm/*/lib path would be added
implicitly and would take precedence over the correct
/usr/lib/llvm/*/lib64 path if supplied by user. Since lib contains
32-bit LLVM/Clang libraries, it would break the 64-bit apps trying
to link against them.

[MaskRay]

Is addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths); from D108286 the issue?

If yes, I lean toward reverting D108286 and possibly removing LIBCXX_LIBDIR_SUFFIX as a CMake configuration (#libc_vendors).

The multilib style lib32 lib64 are more about glibc and GCC library search paths. llvm-project runtime libraries (including libc++) do not necessarily use that style.
In addition, we are shifting to multiarch style hierarchies which render lib32/lib64 unnecessary.

[mgorny]

In D122424#3406275, @MaskRay wrote:

Is addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths); from D108286 the issue?

No, the issue is actually the hardcoded lib directory that breaks multilib builds when using LLD.

[MaskRay]

Obsoleted by D122444 (Hope I am not mistaken)

clang/lib/Driver/ToolChains/Linux.cpp

#endif
   // Similar to the logic for GCC above, if we are currently running Clang
   // inside of the requested system root, add its parent library path to those
   // searched.
   // FIXME: It's not clear whether we should use the driver's installed
   // directory ('Dir' below) or the ResourceDir.
   if (StringRef(D.Dir).startswith(SysRoot)) {
     // Even if OSLibDir != "lib", this is needed for Clang in the build
     // directory (not installed) to find libc++.
+    // Use the presence of build-specific files to detect whether we are
+    // dealing with the build tree and avoid adding 32-bit system "lib"
+    // directory otherwise.
+    if (D.getVFS().exists(D.Dir + "/../CMakeCache.txt"))
       addPathIfExists(D, D.Dir + "/../lib", Paths);
     if (OSLibDir != "lib")
       addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
   }
 
   if (!D.OverlayToolChainPath.empty()) {
     addPathIfExists(D, ExtraPath + "/lib", Paths);
     addPathIfExists(D, ExtraPath + "/usr/lib", Paths);
   }