GDB: Allow passing regcache to resolve_dynamic_type ()

GDB: Allow passing regcache to resolve_dynamic_type ()

When reg_buffer::initialize_variable_size_registers () is called, there
may not be any frame available yet.  On the other hand, the reg_buffer
should already have any register values needed to calculate target
description parameter values and therefore resolve dynamic types.

So allow passing a regcache to resolve_dynamic_type (), and use it in
dwarf2_evaluate_property () when processing a target description
parameter property.

Because initialize_variable_size_registers () is a reg_buffer method,
dwarf2_evaluate_property () can't call tdesc_parameter_value () which is
a readable_regcache method.  So we need to add a
collect_tdesc_parameter () method to reg_buffer and check that the
collected parameter is valid.