Rename demangle.c to gdb-demangle.c, and some cleanup

Rename demangle.c to gdb-demangle.c, and some cleanup

In addition to renaming demangle.c to match the header file naming,
this also makes is_cplus_marker return a bool and removes a duplicate
declaration of "bool demangle" from symtab.h.

gdb/ChangeLog:

2019-11-21  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Update.
	* demangle.c: Rename to...
	* gdb-demangle.c: ..this.
	(is_cplus_marker): Change return type to bool.
	(_initialize_demangler): Rename to...
	(_initialize_gdb_demangle): ...this.
	* gdb-demangle.h (is_cplus_marker): Change return type to bool.
	* symtab.h (demangle): Remove declaration; instead include
	gdb-demangle.h.

Change-Id: I83c3b3f7ee71b2bf6f5b5d0f9eb1d4b5208f2a97