gdb: Convert la_filename_extensions to a method

gdb: Convert la_filename_extensions to a method

Convert language_data::la_filename_extensions member variable to a
virtual method language_defn::filename_extensions.

The new method returns a vector of filename extensions, which means
that where previously we needed a NULL marker on the end of the list,
we can now discard this.

All of the old arrays that contained the extensions now become static
data within each languages filename_extensions method.

I've updated the single use of the filename_extensions method to make
use of this method returning a vector.  And, just in case anyone
accidentally adds a NULL marked into a languages extensions list, I've
added a new assert in add_filename_language (symtab.c) to catch this.

gdb/ChangeLog:

	* ada-lang.c (ada_extensions): Delete, moved into
	ada_language::filename_extensions.
	(ada_language_data): Remove la_filename_extensions initializer.
	(ada_language::filename_extensions): New member function.
	* c-lang.c (c_extensions): Delete, moved into
	c_language::filename_extensions.
	(c_language_data): Remove la_filename_extensions initializer.
	(c_language::filename_extensions): New member function.
	(cplus_extensions): Delete, moved into
	cplus_language::filename_extensions.
	(cplus_language_data): Remove la_filename_extensions initializer.
	(cplus_language::filename_extensions): New member function.
	(asm_extensions): Delete, moved into
	asm_language::filename_extensions.
	(asm_language_data): Remove la_filename_extensions initializer.
	(asm_language::filename_extensions): New member function.
	(minimal_language_data): Remove la_filename_extensions
	initializer.
	* d-lang.c (d_extensions): Delete, moved into
	d_language::filename_extensions.
	(d_language_data): Remove la_filename_extensions initializer.
	(d_language::filename_extensions): New member function.
	* f-lang.c (f_extensions): Delete, moved into
	f_language::filename_extensions.
	(f_language_data): Remove la_filename_extensions initializer.
	(f_language::filename_extensions): New member function.
	* go-lang.c (go_language_data): Remove la_filename_extensions
	initializer.
	* language.c (add_set_language_command): Update now that
	filename_extensions returns a vector.
	(unknown_language_data): Remove la_filename_extensions
	initializer.
	(auto_language_data): Likewise.
	* language.h (language_data): Remove la_filename_extensions field.
	(language_defn::filename_extensions): New member function.
	* m2-lang.c (m2_language_data): Remove la_filename_extensions
	initializer.
	* objc-lang.c (objc_extensions): Delete, moved into
	objc_language::filename_extensions.
	(objc_language_data): Remove la_filename_extensions initializer.
	(objc_language::filename_extensions): New member function.
	* opencl-lang.c (opencl_language_data): Remove
	la_filename_extensions initializer.
	* p-lang.c (pascal_extensions): Delete, moved into
	pascal_language::filename_extensions.
	(pascal_language_data): Remove la_filename_extensions initializer.
	(pascal_language::filename_extensions): New member function.
	* rust-lang.c (rust_extensions): Delete, moved into
	rust_language::filename_extensions.
	(rust_language_data): Remove la_filename_extensions initializer.
	(rust_language::filename_extensions): New member function.
	* symfile.c (add_filename_language): Add new assert.