Tighten test regexes checking for __attribute__((thiscall)) on function types.

The tests were perhaps made too relaxed in r197164 when we switched to the new
MinGW ABI. This makes sure we check explicitly for an optional thiscall
attribute and nothing else.

We should still look into whether we should print these attributes at all in
these cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197252 91177308-0d34-0410-b5e6-96231b3b80d8