Avoid testing for a particular choice of resource dir.

Without this patch, this test was accidentally testing that
CLANG_RESOURCE_DIR, CLANG_LIBDIR_SUFFIX, and CLANG_VERSION_STRING
were set to a particular set of values.

The test was also getting pretty hairy since it was attempting to craft
a regular expression that covered "all" possible combinations of
settings for these configure-time constants.

Clean it up by directly capturing the resource directory in a FileCheck
variable.

llvm-svn: 227310