[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

lib/IR/InlineAsm.cpp

@@ -91,6 +91,10 @@ bool InlineAsm::ConstraintInfo::Parse(StringRef Str,
   if (*I == '~') {
     Type = isClobber;
     ++I;
+
+    // '{' must immediately follow '~'.
+    if (I != E && *I != '{')
+      return true;
   } else if (*I == '=') {
     ++I;
     Type = isOutput;