More strict error checking in parseSpecifier + simplified code.

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.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168991 91177308-0d34-0410-b5e6-96231b3b80d8