Stop crashing on empty asm input constraints

An empty string for an ASM input constraint is invalid, and will crash
during clang CodeGen.  Change TargetInfo::validateInputConstraint to
reject an empty string.

<rdar://problem/15552191>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197362 91177308-0d34-0410-b5e6-96231b3b80d8