Fix crash if delayed template parsing meets an erroneous trailing return type.
Based on a patch and test by Stephan Tolksdorf! Refactoring and fixing adjacent
brokenness by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203733 91177308-0d34-0410-b5e6-96231b3b80d8