More strict error checking in parseSpecifier + simplified code.

For example, don't allow empty strings to be passed to getInt.

Move asserts inside parseSpecifier. (One day we may want to pass parse
error messages to the user - from LLParser - instead of using asserts,
but keep the code simple until then. There have been an attempt to do
this. See r142288, which got reverted, and r142605.)

llvm-svn: 168991