Parse: Don't crash during parsing if we lack a simple-type-specifier

Summary:
Parsing cast expressions during error recovery can put us in a bad
state.  Check to see if the token for a simple-type-specifier makes
sense before further parsing.

Fixes PR17255.

Reviewers: rsmith, doug.gregor, CornedBee, eli.friedman

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1696

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191159 91177308-0d34-0410-b5e6-96231b3b80d8