Fix SEGV in InlineAsm::ConstraintInfo::Parse.

Summary:
Fixes bug 24646. Previous code was not checking if an index into a vector
was valid, resulting in a SEGV. Fixed by assuming the construct can't
be parsed when given this input.

Reformat and add test.

Differential Revision: http://reviews.llvm.org/D12539

llvm-svn: 246774