Perform "re lexing" on string list values rathern than on text.

Perform "re lexing" on string list values rathern than on text.

Previously, we would pass original strings back to the original lexer
whenever we needed to re-lex something, (such as an expanded macro or
a macro argument). Now, we instead parse the macro or argument
originally to a string list, and then re-lex by simply returning each
string from this list in turn.

We do this in the recently added glcpp_parser_lex function that sits
on top of the lower-level glcpp_lex that only deals with text.

This doesn't change any behavior (at least according to the existing
test suite which all still passes) but it brings us much closer to
being able to "finalize" an unexpanded macro as required by the
specification.