fix PR symtab/15028

fix PR symtab/15028

This fixes some derivation.exp regressions with "dwz -m".

The bug here is that the imported PU is given language_minimal.
However, it ought to be C++.

The "pretend language" machinery exists to solve this problem, but it
wasn't handled in process_psymtab_comp_unit.  So, this patch adds it
there.

Built and regtested, both normally and using "dwz -m", on x86-64
Fedora 18.

	PR symtab/15028:
	* dwarf2read.c (struct process_psymtab_comp_unit_data): New.
	(process_psymtab_comp_unit_reader): Use it.
	(process_psymtab_comp_unit): Update.  Add "pretend_language"
	argument.
	(dwarf2_build_psymtabs_hard): Update.
	(scan_partial_symbols): Pass CU's language to
	process_psymtab_comp_unit.