Fix dwarf_expr_context method regressions

Fix dwarf_expr_context method regressions

This fixes some regressions found in the patch to convert
dwarf_expr_context to use methods.  Specifically:

* get_base_type could erroneously throw; this was rewritten to move
  the size checks into the only spot needing them.
* Previously the "symbol needs frame" implementation reused th
  "cfa" function for the get_frame_pc slot; this reimplements
  it under the correct name.
* Not enough members were saved and restored in one implementation
  of push_dwarf_reg_entry_value; this patch fixes this oversight
  and also takes the opportunity to remove an extraneous structure
  definition.

2016-11-02  Tom Tromey  <tom@tromey.com>

	* dwarf2loc.c (dwarf_evaluate_loc_desc::get_base_type): Rename
	from impl_get_base_type.  Rewrite.
	(struct dwarf_expr_baton): Remove.
	(dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value): Save and
	restore more fields.
	(symbol_needs_eval_context::get_frame_pc): New method.
	* dwarf2expr.h (dwarf_expr_context::get_base_type): Now public,
	virtual.
	(dwarf_expr_context::impl_get_base_type): Remove.
	* dwarf2expr.c (dwarf_expr_context::get_base_type): Remove.