Remove dwarf2_per_cu_data::text_offset

This method simply returns the text offset of the objfile associated to
the dwarf2_per_cu_data object.  Since dwarf2_per_cu_data objects are
going to become objfile-independent, we can't keep this method.  This
patch removes it.

Existing callers need to figure out the in the context of which objfile
this is being used, and call text_offset on it.  Typically, this comes
from a symbol baton, where we store the corresponding
dwarf2_per_objfile.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_cu_data) <text_offset>:
	Remove.
	* dwarf2/read.c (dwarf2_per_cu_data::text_offset): Remove.
	* dwarf2/loc.c (dwarf2_find_location_expression): Update.
	(dwarf2_compile_property_to_c): Update.
	(dwarf2_compile_expr_to_ax): Add dwarf2_per_objfile parameter,
	use text offset from objfile.
	(locexpr_tracepoint_var_ref): Update.
	(locexpr_generate_c_location): Update.
	(loclist_describe_location): Update.
	(loclist_tracepoint_var_ref): Update.
	* dwarf2/compile.h (compile_dwarf_bounds_to_c): Add
	dwarf2_per_objfile parameter.
	* dwarf2/loc2c.c (do_compile_dwarf_expr_to_c): Likewise,
	use text offset from objfile.
	(compile_dwarf_expr_to_c): Add dwarf2_per_objfile parameter.

Change-Id: I56b01ba294733362a3562426a96d48ae051a776f