Use symbol_set_language to set a symbol's language

Use symbol_set_language to set a symbol's language

Instead of using SYMBOL_LANGUAGE (sym) = foo.

Having only a single way to set a symbol's language is clearer and this
is also a requirement for making set_language a member function.

gdb/ChangeLog:

2019-12-15  Christian Biesinger  <cbiesinger@google.com>

	* ada-exp.y (write_ambiguous_var): Call symbol_set_language to
	set the language of sym.
	* language.c (language_alloc_type_symbol): Likewise.

Change-Id: I85338ea2e4121155f2da222fe0aa6b7d3ffe26f7