Fix parsing certain kinds of strings in the MS section pragmas

We were crashing on the relevant test case inputs.  Also, refactor this
code a bit so we can report failure and slurp the pragma tokens without
returning a diagnostic id.  This is more consistent with the rest of the
parser and sema code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213337 91177308-0d34-0410-b5e6-96231b3b80d8