Rename "read_reg" into "read_addr_from_reg" in struct dwarf_expr_context_funcs

Rename "read_reg" into "read_addr_from_reg" in struct dwarf_expr_context_funcs

This is to help make it slightly clearer how this method is expected
to extract data from the given register.

gdb/ChangeLog:

        * dwarf2expr.h (struct dwarf_expr_context_funcs)
        <read_addr_from_reg>: Renames "read_reg".
        * dwarf2-frame.c (read_addr_from_reg): Renames "read_reg".
        Adjust comment.
        (dwarf2_frame_ctx_funcs, execute_stack_op, dwarf2_frame_cache):
        Use read_addr_from_reg in place of read_reg.
        * dwarf2expr.c (execute_stack_op): Use read_addr_from_reg
        in place of read_reg.
        * dwarf2loc.c (dwarf_expr_read_addr_from_reg): Renames
        dwarf_expr_read_reg.
        (dwarf_expr_ctx_funcs): Replace dwarf_expr_read_reg
        with dwarf_expr_read_addr_from_reg.
        (needs_frame_read_addr_from_reg): Renames needs_frame_read_reg.
        (needs_frame_ctx_funcs): Replace needs_frame_read_reg with
        needs_frame_read_addr_from_reg.