Parse: Avoid crashing on unterminated top-level asm strings

When parsing invalid top-level asm statements, we were ignoring the
return code of the SkipUntil we used for recovery. This led to crashes
when we hit the end of file and tried to continue parsing anyway.

This fixes the crash and adds a couple of tests for parsing related
problems.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196961 91177308-0d34-0410-b5e6-96231b3b80d8