gdb: Rename language_defn::demangle

gdb: Rename language_defn::demangle

GDB already has a global symbol `demangle` (a boolean), having a
language method called `demangle` is not a good idea as we often want
to reference `demangle` the control variable inside `demangle` the
member function.

This commit renames `demangle` the member function to
`demangle_symbol`.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language::demangle): Rename to...
	(ada_language::demangle_symbol): ...this.
	* c-lang.c (cplus_language::demangle): Rename to...
	(cplus_language::demangle_symbol): ...this.
	* d-lang.c (d_language::demangle): Rename to...
	(d_language::demangle_symbol): ...this.
	* f-lang.c (f_language::demangle): Rename to...
	(f_language::demangle_symbol): ...this.
	* go-lang.c (go_language::demangle): Rename to...
	(go_language::demangle_symbol): ...this.
	* language.c (language_demangle): Update call to demangle_symbol.
	(auto_or_unknown_language::demangle): Rename to...
	(auto_or_unknown_language::demangle_symbol): ...this.
	* language.h (language_defn::demangle): Rename to...
	(language_defn::demangle_symbol): ...this.
	* objc-lang.c (objc_language::demangle): Rename to...
	(objc_language::demangle_symbol): ...this.
	* rust-lang.c (rust_language::demangle): Rename to...
	(rust_language::demangle_symbol): ...this.