Bug 1439767 - add .mozbuild clang path to the toolchain search path on Windows; r=nalexander

Doing this means compiling with clang-cl is just `export CC=clang-cl` in
somebody's mozconfig, rather than something more elaborate.

build/moz.configure/toolchain.configure

@@ -646,6 +646,14 @@ def toolchain_search_path(vc_compiler_path):
     if vc_compiler_path:
         result = [os.environ.get('PATH')]
         result.extend(vc_compiler_path)
+
+        # Also add in the location to which `mach bootstrap` or
+        # `mach artifact toolchain` installs clang.
+        mozbuild_state_dir = os.environ.get('MOZBUILD_STATE_PATH',
+                                            os.path.expanduser(os.path.join('~', '.mozbuild')))
+        bootstrap_clang_path = os.path.join(mozbuild_state_dir, 'clang', 'bin')
+        result.append(bootstrap_clang_path)
+
         # We're going to alter PATH for good in windows.configure, but we also
         # need to do it for the valid_compiler() check below.
         os.environ['PATH'] = os.pathsep.join(result)