[libcxx] Support runtimes and monorepo locations for tests

The test configuration support currently searches for libc++ sources
in <ROOT>/projects/libcxx. This change also additionally searches
<ROOT>/runtimes/libcxx (so called runtimes layout) and <ROOT>/libcxx
(monorepo layout).

This matches the logic we already use in CMake, for example:
https://github.com/llvm/llvm-project/blob/6fd4e7f/libcxx/CMakeLists.txt#L148

When the monorepo becomes the only supported layout in the future,
we can simplify this logic again.

Differential Revision: https://reviews.llvm.org/D57776

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353600 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/test/config.py

@@ -281,9 +281,15 @@ class Configuration(object):
         self.project_obj_root = self.get_lit_conf('project_obj_root')
         self.libcxx_obj_root = self.get_lit_conf('libcxx_obj_root')
         if not self.libcxx_obj_root and self.project_obj_root is not None:
-            possible_root = os.path.join(self.project_obj_root, 'projects', 'libcxx')
-            if os.path.isdir(possible_root):
-                self.libcxx_obj_root = possible_root
+            possible_roots = [
+                os.path.join(self.project_obj_root, 'libcxx'),
+                os.path.join(self.project_obj_root, 'projects', 'libcxx'),
+                os.path.join(self.project_obj_root, 'runtimes', 'libcxx'),
+            ]
+            for possible_root in possible_roots:
+                if os.path.isdir(possible_root):
+                    self.libcxx_obj_root = possible_root
+                    break
             else:
                 self.libcxx_obj_root = self.project_obj_root