libc++ testing: allow to provide a path for use_system_cxx_lib

As we're trying to setup testing / bots for all shipping version of libc++
on macOS/iOS, we'll need to be able to pass a path to where to find the
dylib for each previous version of the OS.

Differential Revision: https://reviews.llvm.org/D31486

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299053 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/test/target_info.py

@@ -142,6 +142,8 @@ class DarwinLocalTI(DefaultTargetInfo):
         library_paths = []
         # Configure the library path for libc++
         if self.full_config.use_system_cxx_lib:
+            if (os.path.isdir(str(self.full_config.use_system_cxx_lib))):
+                library_paths += [self.full_config.use_system_cxx_lib]
             pass
         elif self.full_config.cxx_runtime_root:
             library_paths += [self.full_config.cxx_runtime_root]