ExtractAPI: Use %clang_cc1 and -verify in enum.c

[dexonsmith]

Summary

Fix one test (enum.c) in ExtractAPI to use %clang_cc1 and -verify
instead of calling the full driver and FileCheck. This is an example for
my comment from https://reviews.llvm.org/D121873.

[dang]

LGTM!

clang/test/ExtractAPI/enum.c

 // RUN: rm -rf %t
 // RUN: split-file %s %t
 // RUN: sed -e "s@INPUT_DIR@%{/t:regex_replacement}@g" \
 // RUN: %t/reference.output.json.in >> %t/reference.output.json
-// RUN: %clang -extract-api -target arm64-apple-macosx \
-// RUN: %t/input.h -o %t/output.json | FileCheck -allow-empty %s
+// RUN: %clang_cc1 -extract-api -triple arm64-apple-macosx \
+// RUN:   -x c-header %t/input.h -o %t/output.json -verify
 
 // Generator version is not consistent across test runs, normalize it.
 // RUN: sed -e "s@\"generator\": \".*\"@\"generator\": \"?\"@g" \
 // RUN: %t/output.json >> %t/output-normalized.json
 // RUN: diff %t/reference.output.json %t/output-normalized.json
 
-// CHECK-NOT: error:
-// CHECK-NOT: warning:
-
 //--- input.h
 /// Kinds of vehicles
 enum Vehicle {
   Bicycle,
   Car,
   Train, ///< Move this to the top! -Sheldon
   Ship,
   Airplane,
 };
 
 enum Direction : unsigned char {
   North = 0,
   East,
   South,
   West
 };
 
 enum {
   Constant = 1
 };
 
 enum {
   OtherConstant = 2
 };
+// expected-no-diagnostics
 
 //--- reference.output.json.in
 {
   "metadata": {
     "formatVersion": {
       "major": 0,
       "minor": 5,
       "patch": 3