[clang] Revert changes to prefer the toolchain-provided libc++.dylib

This patch reverts the change that made clang prefer the toolchain
libc++.dylib when there is one (#170303), and the subsequent test
workaround we landed to fix bots (#170912).

We are seeing some failure on macOS LLDB bots that need to be
investigated, and that will require more time than I can spare
before the end of today.

This reverts commits bad1a88 and 190b8d0.

Author: ldionne

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2846,49 +2846,11 @@ void AppleMachO::AddCXXStdlibLibArgs(const ArgList &Args,
   CXXStdlibType Type = GetCXXStdlibType(Args);
 
   switch (Type) {
-  case ToolChain::CST_Libcxx: {
-    // On Darwin, we prioritize a libc++ located in the toolchain to a libc++
-    // located in the sysroot. Unlike the driver for most other platforms, on
-    // Darwin we do that by explicitly passing the library path to the linker
-    // to avoid having to add the toolchain's `lib/` directory to the linker
-    // search path, which would make other libraries findable as well.
-    //
-    // Prefering the toolchain library over the sysroot library matches the
-    // behavior we have for headers, where we prefer headers in the toolchain
-    // over headers in the sysroot if there are any. Note that it's important
-    // for the header search path behavior to match the link-time search path
-    // behavior to ensure that we link the program against a library that
-    // matches the headers that were used to compile it.
-    //
-    // Otherwise, we end up compiling against some set of headers and then
-    // linking against a different library (which, confusingly, shares the same
-    // name) which may have been configured with different options, be at a
-    // different version, etc.
-    SmallString<128> InstallLib = llvm::sys::path::parent_path(getDriver().Dir);
-    llvm::sys::path::append(InstallLib, "lib"); // <install>/lib
-    auto Link = [&](StringRef Library) {
-      SmallString<128> Shared(InstallLib);
-      llvm::sys::path::append(Shared,
-                              SmallString<4>("lib") + Library + ".dylib");
-      SmallString<128> Static(InstallLib);
-      llvm::sys::path::append(Static, SmallString<4>("lib") + Library + ".a");
-      SmallString<32> Relative("-l");
-      Relative += Library;
-
-      if (getVFS().exists(Shared)) {
-        CmdArgs.push_back(Args.MakeArgString(Shared));
-      } else if (getVFS().exists(Static)) {
-        CmdArgs.push_back(Args.MakeArgString(Static));
-      } else {
-        CmdArgs.push_back(Args.MakeArgString(Relative));
-      }
-    };
-
-    Link("c++");
+  case ToolChain::CST_Libcxx:
+    CmdArgs.push_back("-lc++");
     if (Args.hasArg(options::OPT_fexperimental_library))
-      Link("c++experimental");
+      CmdArgs.push_back("-lc++experimental");
     break;
-  }
 
   case ToolChain::CST_Libstdcxx:
     // Unfortunately, -lstdc++ doesn't always exist in the standard search path;

clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++.dylib

@@ -92,7 +92,7 @@
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
-// Make sure that using -nostdinc, -nostdinc++ or -nostdlibinc will drop both the toolchain
+// Make sure that using -nostdinc, -nostdinc++ or -nostdlib will drop both the toolchain
 // C++ include path and the sysroot one.
 //
 // RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -116,7 +116,7 @@
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     -stdlib=platform \
 // RUN:     -nostdinc++ \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINCXX %s
 // CHECK-LIBCXX-NOSTDINCXX: "-cc1"

clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++experimental.a

@@ -18,8 +18,6 @@
 // CHECK: -fexperimental-library
 
 // Depending on the stdlib in use, we should (or not) pass -lc++experimental.
-// Note that we don't check for `-lc++experimental` specifically, since some targets
-// like Darwin pass the path to the library explicitly instead of using `-lx`.
-// CHECK-LIBCXX: c++experimental
-// CHECK-LIBSTDCXX-NOT: c++experimental
-// CHECK-NOSTDLIB-NOT: c++experimental
+// CHECK-LIBCXX: -lc++experimental
+// CHECK-LIBSTDCXX-NOT: -lc++experimental
+// CHECK-NOSTDLIB-NOT: -lc++experimental