Swift: replace $(CC) with clang

On macOS `$(CC)` points to a wrapper that requires `DEVELOPER_DIR` to be
set in the environment. Using `clang` is slightly less generic, but
that's our default any way. Even if we do set a different clang version
somewhere, the selected version of GCC would not change, and the test
is targeting that.

Author: redsun82

swift/extractor/infra/file/BUILD.bazel

@@ -18,12 +18,11 @@ genrule(
     # see if https://cplusplus.github.io/LWG/issue3657 is fixed with the current compiler or not
     # if fixed, PathHash.h.workaround will not compile
     cmd = "\n".join([
-        "if $(CC) -c -x c++ -std=c++17 -Wno-pragma-once-outside-header \\",
+        "if clang -c -x c++ -std=c++17 -Wno-pragma-once-outside-header \\",
         "    $(rootpath PathHash.h.workaround) -o /dev/null &> /dev/null; then",
         "  cp $(rootpath PathHash.h.workaround) $@",
         "else",
         "  cp $(rootpath PathHash.h.fixed) $@",
         "fi",
     ]),
-    toolchains = ["@bazel_tools//tools/cpp:current_cc_host_toolchain"],
 )