ARM: fix AST for __builtin_arm_strex call

The AST was constructed so that this builtin returned the default BoolTy and
since I'd opted for custom SemaChecking, I should have set it properly at that
point.

This caused an assertion failure when the types didn't match up with what we
generated. This makes it return an IntTy, which is as good as anything.

llvm-svn: 193606