Improve the diagnostics of the number-reading preprocessor directives.

The GNU line marker directive was sharing code with the #line directive, but some of the warnings/errors were reporting as #line directive diagnostics in both cases.

Previously:
#line 11foo1   ==> "#line directive requires a simple digit sequence"
# 11foo1       ==> "#line directive requires a simple digit sequence"

Now, we get:
#line 11foo1   ==> "#line directive requires a simple digit sequence"
# 11foo1       ==> "GNU line marker directive requires a simple digit sequence"