Return void from buildsym_compunit::push_context

There is one caller that uses the result of
buildsym_compunit::push_context.  This patch changes this method to
return void and changes that spot to instead call a new methods on
buildsym_compunit.

This patch also removes the get_current_context_stack method in favor
of a new method that checks the exact condition needed by the one
caller.

This patch enables a subsequent cleanup; in particular now the
'context_stack' type isn't used outside of buildsym.

Approved-By: Simon Marchi <simon.marchi@efficios.com>