[libcxx] Support runtimes and monorepo locations for tests

[phosek]

Summary

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.

[phosek]

After this and D57775 lands, together with all the lit and CMake changes that landed in the last few days, we'll be able to run libunwind, libc++abi and libc++ tests in the monorepo layout using the runtimes build with compiler-rt builtins and libunwind as the unwinder \o/

[phosek]

Ping?

[smeenai]

LGTM

libcxx/trunk/utils/libcxx/test/config.py

class Configuration(object):
     def configure_src_root(self):
         self.libcxx_src_root = self.get_lit_conf(
             'libcxx_src_root', os.path.dirname(self.config.test_source_root))
 
     def configure_obj_root(self):
         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')
+            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
 
     def configure_cxx_library_root(self):
         self.cxx_library_root = self.get_lit_conf('cxx_library_root',
                                                   self.libcxx_obj_root)
         self.cxx_runtime_root = self.get_lit_conf('cxx_runtime_root',
                                                    self.cxx_library_root)