diagnostics: Use an inline function rather than hardcoding <built-in> string

The string "<built-in>" is hard-coded in several places throughout the
diagnostics code, and in some of those places, it is used incorrectly with
respect to internationalization. (Comparing a translated string to an
untranslated string.) The error is not currently observable in any output GCC
actually produces, hence no testcase added here, but it's worth fixing, and
also, I am shortly going to add a new such string and want to avoid hardcoding
that one in similar places.

gcc/c-family/ChangeLog:

	* c-opts.cc (c_finish_options): Use special_fname_builtin () rather
	than a hard-coded string.

gcc/ChangeLog:

	* diagnostic.cc (diagnostic_get_location_text): Use
	special_fname_builtin () rather than a hardcoded string (which was
	also incorrectly left untranslated previously.)
	* input.cc (special_fname_builtin): New function.
	(expand_location_1): Use special_fname_builtin () rather than a
	hard-coded string.
	(test_builtins): Likewise.
	* input.h (special_fname_builtin): Declare.

gcc/fortran/ChangeLog:

	* cpp.cc (gfc_cpp_init): Use special_fname_builtin () rather than a
	hardcoded string (which was also incorrectly left untranslated
	previously.)
	* error.cc (gfc_diagnostic_build_locus_prefix): Likewise.
	* f95-lang.cc (gfc_init): Likewise.