[bazel] Add lib/Basic/BuiltinTargetFeatures.h to clang:basic hdrs.

This header is included by
clang/unittests/CodeGen/CheckTargetFeaturesTest.cpp
so it needs to be exposed here to make it visible.

Author: slackito

utils/bazel/llvm-project-overlay/clang/BUILD.bazel

@@ -403,6 +403,7 @@ cc_library(
     ]),
     hdrs = glob([
         "include/clang/Basic/*.h",
+        "lib/Basic/BuiltinTargetFeatures.h",
     ]),
     copts = [
         "-DHAVE_VCS_VERSION_INC",