[InlineAsm] Improve error messages for invalid constraint strings

InlineAsm constraint string verification can fail for many reasons,
but used to always print a generic "invalid type for inline asm
constraint string" message -- which is especially confusing if
the actual error is unrelated to the type, e.g. a failure to parse
the constraint string.

Change the verify API to return an Error with a more specific
error message, and print that in the IR parser.