[analyzer] scan-build: match whitespace instead of word boundaries around flags.

Because neither ' ' nor '-' is alphanumeric, \b won't match between them!
Since in this case we know our output is coming from a -### invocation,
we should always have spaces on both sides of the flag we're trying to match,
"-cc1".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204356 91177308-0d34-0410-b5e6-96231b3b80d8