gdb: Convert la_name_of_this to a method

Convert language_data::la_name_of_this member variable to a virtual
method language_defn::name_of_this.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Remove la_name_of_this
	initializer.
	* ax-gdb.c (gen_expr): Update call to name_of_this.
	* c-exp.y (classify_name): Likewise.
	* c-lang.c (c_language_data): Remove la_name_of_this initializer.
	(cplus_language_data): Likewise.
	(cplus_language::name_of_this): New member function.
	(asm_language_data): Remove la_name_of_this initializer.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	(d_language::name_of_this): New member function.
	* expprint.c (print_subexp_standard): Update call to name_of_this.
	* f-lang.c (f_language_data): Remove la_name_of_this initializer.
	* go-lang.c (go_language_data): Likewise.
	* language.c (unknown_language_data): Likewise.
	(unknown_language::name_of_this): New member function.
	(auto_language_data): Remove la_name_of_this initializer.
	(auto_language::name_of_this): New member function.
	* language.h (language_data): Delete la_name_of_this member
	variable.
	(language_defn::name_of_this): New member function.
	* m2-lang.c (m2_language_data): Remove la_name_of_this
	initializer.
	* objc-lang.c (objc_language_data): Likewise.
	(objc_language::name_of_this): New member function.
	* opencl-lang.c (opencl_language_data): Remove la_name_of_this
	initializer.
	* p-lang.c (pascal_language_data): Likewise.
	(pascal_language::name_of_this): New member function.
	* rust-lang.c (rust_language_data): Remove la_name_of_this
	initializer.
	* symtab.c (lookup_language_this): Update call to name_of_this.
	(lookup_symbol_aux): Likewise.
	* valops.c (value_of_this): Likewise.