implement PR4077: [Linux kernel] inscrutable error on inline asm input/output constraint mismatch

Before we emitted:

$ clang t.c -S -m64 
llvm: error: Unsupported asm: input constraint with a matching output constraint of incompatible type!

Now we produce:
$ clang t.c -S -m64 
t.c:5:40: error: unsupported inline asm: input with type 'unsigned long' matching output with type 'int'
  asm volatile("foo " : "=a" (a) :"0" (b));
                             ~~~      ~^~

llvm-svn: 70142