gdb: Convert language la_read_var_value field to a method

gdb: Convert language la_read_var_value field to a method

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

An interesting aspect of this change is that the implementation of
language_defn::read_var_value is actually in findvar.c.  This is
partly historical, the new language_defn::read_var_value is a rename
of default_read_var_value, which was already in that file, but also,
that is the file that contains the helper functions needed by the
read_var_value method, so it makes sens that the method implementation
should continue to live there (I think).

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_read_var_value): Delete function, move
	implementation to...
	(ada_language::read_var_value): ...here.
	(ada_language_data): Delete la_read_var_value initializer.
	* c-lang.c (c_language_data): Likewise.
	(cplus_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_language_data): Likewise.
	* findvar.c (default_read_var_value): Rename to...
	(language_defn::read_var_value): ...this.
	* findvar.c (read_var_value): Update header comment, and change to
	call member function instead of function pointer.
	* go-lang.c (go_language_data): Likewise.
	* language.c (unknown_language_data): Delete la_read_var_value
	initializer.
	(auto_language_data): Likewise.
	* language.h (struct language_data): Delete la_read_var_value
	field.
	(language_defn::read_var_value): New member function.
	(default_read_var_value): Delete declaration.
	* m2-lang.c (m2_language_data): Delete la_read_var_value
	initializer.
	* objc-lang.c (objc_language_data): Likewise.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.
	* value.h (default_read_var_value): Delete declaration.