Restore that for now, we're not GenToo ...

https://reviews.llvm.org/D148945
https://reviews.llvm.org/D122444

Index: llvm-toolchain-snapshot_19~++20240424094944+78ebaa2d798f/clang/lib/Driver/ToolChains/Linux.cpp
===================================================================
--- llvm-toolchain-snapshot_19~++20240424094944+78ebaa2d798f.orig/clang/lib/Driver/ToolChains/Linux.cpp
+++ llvm-toolchain-snapshot_19~++20240424094944+78ebaa2d798f/clang/lib/Driver/ToolChains/Linux.cpp
@@ -345,6 +345,13 @@ Linux::Linux(const Driver &D, const llvm
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
+  // 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).starts_with(SysRoot) &&
+      D.getVFS().exists(D.Dir + "/../lib/libc++.so"))
+    addPathIfExists(D, D.Dir + "/../lib", Paths);
+
   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);
   addPathIfExists(D, concat(SysRoot, "/usr/lib"), Paths);
 }