gdb: Convert language_data::la_case_sensitivity to a method

gdb: Convert language_data::la_case_sensitivity to a method

Convert language_data::la_case_sensitivity member variable to a virtual
method language_defn::case_sensitivity.

This is mostly straight forward.  The only slight problem is that I
ended up deleting this comment from ada-lang.c:

  /* Yes, Ada is case-insensitive, but that's not quite what this
     means.  */

However, as the comment (which has existed since Ada support was first
added to GDB) doesn't explain _why_ Ada sets case sensitivity to on
despite being a generally case insensitive language, this doesn't
really help me much.

If I understood _why_ the setting doesn't quite mean what it seems to
mean (at least as far as Ada is concerned) then I would extend the
comment on language_defn::case_sensitivity (in language.h) to include
the detail, and note how this impacts Ada.  But as it stands I've just
deleted the comment for now.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Remove la_case_sensitivity
	initializer.
	* c-lang.c (c_language_data): Likewise.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_language_data): Likewise.
	(f_language::case_sensitivity): New member function.
	* go-lang.c (go_language_data): Remove la_case_sensitivity
	initializer.
	* language.c (enum case_mode): Moved here from language.h.
	(case_mode): Make static.
	(show_case_command): Update for case_sensitivity being a method.
	(set_case_command): Likewise.
	(set_range_case): Likewise.
	(unknown_language_data): Remove la_case_sensitivity initializer.
	(auto_language_data): Likewise.
	* language.h (case_mode): Delete, move enum declaration to
	language.c.
	(language_data): Delete la_case_sensitivity field.
	(language_defn::case_sensitivity): New member function.
	* m2-lang.c (m2_language_data): Remove la_case_sensitivity
	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.