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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246774 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed