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

[mehdi_amini]

Summary

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.

[EricWF]

LGTM.

utils/libcxx/test/config.py

def configure_cxx_library_root(self):
                                                    self.cxx_library_root)
 
     def configure_use_system_cxx_lib(self):
         # This test suite supports testing against either the system library or
         # the locally built one; the former mode is useful for testing ABI
         # compatibility between the current headers and a shipping dynamic
         # library.
         # Default to testing against the locally built libc++ library.
-        self.use_system_cxx_lib = self.get_lit_bool('use_system_cxx_lib', False)
+        self.use_system_cxx_lib = self.get_lit_conf('use_system_cxx_lib')
+        if self.use_system_cxx_lib == 'true':
+            self.use_system_cxx_lib = True
+        elif self.use_system_cxx_lib == 'false':
+            self.use_system_cxx_lib = False
+        else:
+            assert os.path.isdir(self.use_system_cxx_lib)
         self.lit_config.note(
             "inferred use_system_cxx_lib as: %r" % self.use_system_cxx_lib)
 
     def configure_apple_availability(self):
         # FIXME doc
         self.with_apple_availability = self.get_lit_conf('with_apple_availability')
         self.lit_config.note(
             "inferred with_apple_availability as: %r" % self.with_apple_availability)
def configure_link_flags_cxx_library_path(self):
                 if self.is_windows and self.link_shared:
                     self.add_path(self.cxx.compile_env, self.cxx_library_root)
             if self.cxx_runtime_root:
                 if not self.is_windows:
                     self.cxx.link_flags += ['-Wl,-rpath,' +
                                             self.cxx_runtime_root]
                 elif self.is_windows and self.link_shared:
                     self.add_path(self.exec_env, self.cxx_runtime_root)
+        elif os.path.isdir(self.use_system_cxx_lib):
+            self.cxx.link_flags += ['-L' + self.use_system_cxx_lib]
+            if not self.is_windows:
+                self.cxx.link_flags += ['-Wl,-rpath,' +
+                                        self.use_system_cxx_lib]
+            if self.is_windows and self.link_shared:
+                self.add_path(self.cxx.compile_env, self.use_system_cxx_lib)
 
     def configure_link_flags_abi_library_path(self):
         # Configure ABI library paths.
         self.abi_library_root = self.get_lit_conf('abi_library_path')
         if self.abi_library_root:
             self.cxx.link_flags += ['-L' + self.abi_library_root]
             if not self.is_windows:
                 self.cxx.link_flags += ['-Wl,-rpath,' + self.abi_library_root]