[Driver][Linux] Remove D.Dir+"/../lib" from default search paths for LLVM_ENABLE_RUNTIMES builds

The rule was added in 2014 to support -stdlib=libc++ and -lc++ without
specifying -L, when D.Dir is not a well-known system library directory like
/usr/lib /usr/lib64. This rule turns out to get in the way with (-m32 for
64-bit clang) or (-m64 for 32-bit clang) for Gentoo :
llvm/llvm-project#54515

Nowadays LLVM_ENABLE_RUNTIMES is the only recommended way building libc++ and
LLVM_ENABLE_PROJECTS=libc++ is deprecated. LLVM_ENABLE_RUNTIMES builds libc++
in D.Dir+"/../lib/${triple}/". The rule is unneeded. Also reverts D108286.

Gentoo uses a modified LLVM_ENABLE_RUNTIMES that installs libc++.so in
well-known paths like /usr/lib64 and /usr/lib which are already covered by
nearby search paths.

Implication: if a downstream package needs something like -lLLVM-15git and uses
libLLVM-15git.so not in a well-known path, it needs to supply -L
D.Dir+"/../lib" explicitly (e.g. via LLVMConfig.cmake), instead of relying on
the previous default search path.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D122444

Author: MaskRay

clang/lib/Driver/ToolChains/Linux.cpp

@@ -309,18 +309,12 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
-  // 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++.
+  // The deprecated -DLLVM_ENABLE_PROJECTS=libcxx configuration installs
+  // libc++.so in D.Dir+"/../lib/". Detect this path.
+  // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
+  if (StringRef(D.Dir).startswith(SysRoot) &&
+      D.getVFS().exists(D.Dir + "/../lib/libc++.so"))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
-    if (OSLibDir != "lib")
-      addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
-  }
 
   if (!D.OverlayToolChainPath.empty()) {
     addPathIfExists(D, ExtraPath + "/lib", Paths);

clang/test/Driver/linux-ld.c

@@ -530,7 +530,6 @@
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-BASIC-LIBCXX-INSTALL: "--sysroot=[[SYSROOT]]"
-// CHECK-BASIC-LIBCXX-INSTALL: "-L[[SYSROOT]]/usr/bin/../lib"
 //
 // Test that we can use -stdlib=libc++ in a build system even when it
 // occasionally links C code instead of C++ code.
@@ -547,7 +546,6 @@
 // CHECK-BASIC-LIBCXX-C-LINK-NOT: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v1"
 // CHECK-BASIC-LIBCXX-C-LINK: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-BASIC-LIBCXX-C-LINK: "--sysroot=[[SYSROOT]]"
-// CHECK-BASIC-LIBCXX-C-LINK: "-L[[SYSROOT]]/usr/bin/../lib"
 //
 // Check multi arch support on Ubuntu 12.04 LTS.
 // RUN: %clang -no-canonical-prefixes %s -no-pie -### -o %t.o 2>&1 \