[update_cc_test_checks.py] Pass the builtin include dir to clang

[arichardson]

Summary

This is required to update tests that make use of builtin headers. To fix
this use the same command expansion as lit does for %clang_cc1. I tested
this by updating clang/test/CodeGen/arm-mve-intrinsics/scalar-shifts.c.
%clang_cc1 will now expand to `clang -cc1 -internal-isystem
$LLVM_BUILD/lib/clang/$VERSION/include -nostdsysteminc`.

[MaskRay]

This makes sense, but can you add a comment in Phabricator, or if it is short, add it to the git description? It helps understanding for posterity.

[arichardson]

In D69611#1727287, @MaskRay wrote:

This makes sense, but can you add a comment in Phabricator, or if it is short, add it to the git description? It helps understanding for posterity.

Sorry, I don't quite understand what you would like me to do. Should I add the comment from the python to the commit message?

[MaskRay]

Add an example expansion of %clang_cc1 to the description.

[arichardson]

Improve commit message

llvm/utils/update_cc_test_checks.py

if args.clang is None:
     if args.llvm_bin is None:
       args.clang = 'clang'
     else:
       args.clang = os.path.join(args.llvm_bin, 'clang')
   if not distutils.spawn.find_executable(args.clang):
     print('Please specify --llvm-bin or --clang', file=sys.stderr)
     sys.exit(1)
 
+  # Determine the builtin includes directory so that we can update tests that
+  # depend on the builtin headers. See get_clang_builtin_include_dir() and
+  # use_clang() in llvm/utils/lit/lit/llvm/config.py.
+  try:
+    builtin_include_dir = subprocess.check_output(
+      [args.clang, '-print-file-name=include']).decode().strip()
+    SUBST['%clang_cc1'] = ['-cc1', '-internal-isystem', builtin_include_dir,
+                           '-nostdsysteminc']
+  except subprocess.CalledProcessError:
+    common.warn('Could not determine clang builtins directory, some tests '
+                'might not update correctly.')
+
   if args.opt is None:
     if args.llvm_bin is None:
       args.opt = 'opt'
     else:
       args.opt = os.path.join(args.llvm_bin, 'opt')
   if not distutils.spawn.find_executable(args.opt):
     # Many uses of this tool will not need an opt binary, because it's only
     # needed for updating a test that runs clang | opt | FileCheck. So we