Implement signedness caching for values, value lists, constants and

constant lists. This is just an internal change to the parser in
preparation for some backwards compatibility code that is to follow.
This will allow things like "uint 4000000000" to retain the unsignedness
of the integer constant as the value moves through the parser. In the
future, all integer types will be signless but parsing "uint" and friends
will be retained for backwards compatibility.

llvm-svn: 31964