[inline asm] Add a check in InlineAsm::ConstraintInfo::Parse to make sure '{'
follows '~' in a clobber constraint string.

Previously llc would hit an llvm_unreachable when compiling an inline-asm
instruction with malformed constraint string "~x{21}". This commit enables
LLParser to catch the error earlier and print a more helpful diagnostic.

rdar://problem/14206559


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217288 91177308-0d34-0410-b5e6-96231b3b80d8