[PATCH v2 1/4] Support for CodeView debugging format

[PATCH v2 1/4] Support for CodeView debugging format

This patch and the following add initial support for Microsoft's
CodeView debugging format, as used by MSVC, to mingw targets.

Note that you will need a recent version of binutils for this to be
useful. The best way to view the output is to run Microsoft's
cvdump.exe, found in their microsoft-pdb repo on GitHub, against the
object files.

gcc/

	* Makefile.in (OBJS): Add dwarf2codeview.o.
	(GTFILES): Add dwarf2codeview.cc
	* config/i386/cygming.h (CODEVIEW_DEBUGGING_INFO): Define.
	* dwarf2codeview.cc: New file.
	* dwarf2codeview.h: New file.
	* dwarf2out.cc: Include dwarf2codeview.h.
	(dwarf2out_finish): Call codeview_debug_finish as needed.
	* flag-types.h (DINFO_TYPE_CODEVIEW): Add enum member.
	(CODEVIEW_DEBUG): Define.
	* flags.h (codeview_debuginfo_p): Proottype.
	* opts.cc (debug_type_names): Add codeview.
	(debug_type_masks): Add CODEVIEW_DEBUG.
	(df_set_names): Add codeview.
	(codeview_debuginfo_p): New function.
	(dwarf_based_debuginfo_p): Add CODEVIEW clause.
	(set_debug_level): Handle CODEVIEW_DEBUG.
	* toplev.cc (process_options): Handle codeview.

gcc/testsuite
	* gcc.dg/debug/codeview/codeview-1.c: New test.
	* gcc.dg/debug/codeview/codeview.exp: New testsuite driver.