Output CodeView function information

Output CodeView function information

Translate DW_TAG_subprogram DIEs into CodeView LF_FUNC_ID types and
S_GPROC32_ID / S_LPROC32_ID symbols.  ld will then transform these into
S_GPROC32 / S_LPROC32 symbols, which map addresses to unmangled function
names.

gcc/
	* dwarf2codeview.cc (enum cv_sym_type): Add new values.
	(struct codeview_symbol): Add function to union.
	(struct codeview_custom_type): Add lf_func_id to union.
	(write_function): New function.
	(write_codeview_symbols): Call write_function.
	(write_lf_func_id): New function.
	(write_custom_types): Call write_lf_func_id.
	(add_function): New function.
	(codeview_debug_early_finish): Call add_function.