gdb: Convert language la_demangle field to a method

gdb: Convert language la_demangle field to a method

This commit changes the language_data::la_demangle function pointer
member variable into a member function of language_defn.

The only slightly "weird" change in this commit is in f-lang.c, where
I have given the Fortran language a demangle method that is identical
to the default language_defn::demangle.  The only reason for this is
to give me somewhere to copy the comment that was previously embedded
within the f_language_data structure.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Delete la_demangle initializer.
	(ada_language::demangle): New member function.
	* c-lang.c (c_language_data): Delete la_demangle initializer.
	(cplus_language_data): Delete la_demangle initializer.
	(cplus_language::demangle): New member function.
	(asm_language_data): Delete la_demangle initializer.
	(minimal_language_data): Delete la_demangle initializer.
	* d-lang.c (d_language_data): Delete la_demangle initializer.
	(d_language::demangle): New member function.
	* f-lang.c (f_language_data): Delete la_demangle initializer.
	(f_language::demangle): New member function.
	* go-lang.c (go_language_data): Delete la_demangle initializer.
	(go_language::demangle): New member function.
	* language.c (language_demangle): Update.
	(unk_lang_demangle): Delete.
	(unknown_language_data): Delete la_demangle initializer.
	(unknown_language::demangle): New member function.
	(auto_language_data): Delete la_demangle initializer.
	(auto_language::demangle): New member function.
	* language.h (language_data): Delete la_demangle field.
	(language_defn::demangle): New function.
	* m2-lang.c (m2_language_data): Delete la_demangle initializer.
	* objc-lang.c (objc_language_data): Delete la_demangle
	initializer.
	(objc_language::demangle): New member function.
	* opencl-lang.c (opencl_language_data): Delete la_demangle
	initializer.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.
	(rust_language::demangle): New member functi