Return unique_ptr from language_defn::get_compile_context

Return unique_ptr from language_defn::get_compile_context

This changes language_defn::get_compile_context to return a
unique_ptr.  This makes the ownership transfer clear.

gdb/ChangeLog
2021-02-05  Tom Tromey  <tom@tromey.com>

	* compile/compile-c-support.c (get_compile_context)
	(c_get_compile_context, cplus_get_compile_context): Change return
	type.
	* language.c (language_defn::get_compile_instance): New method.
	* language.h (language_defn::get_compile_instance): Change return
	type.  No longer inline.
	* c-lang.c (c_language::get_compile_instance): Change return type.
	(cplus_language::get_compile_instance): Change return type.
	* c-lang.h (c_get_compile_context, cplus_get_compile_context):
	Change return type.
	* compile/compile.c (compile_to_object): Update.