Emit .file 0 directive earlier in DWARF 5

Emit .file 0 directive earlier in DWARF 5

When the assembler supports it, the compiler automatically passes --gdwarf-5
to it, which has an interesting side effect: any assembly instruction prior
to the first .file directive defines a new line associated with .file 0 in
the .debug_line section and of course the numbering of these implicit lines
has nothing to do with that of the source code.  This can be problematic in
Ada when we do not generate .file/.loc directives for compiled-generated
functions to avoid too jumpy a debugging experience.

gcc/
	* dwarf2out.c (dwarf2out_assembly_start): Emit .file 0 marker here..
	(dwarf2out_finish): ...instead of here.