gdb: add type parameter to value::allocate_register and add value::allocate_register_lazy

gdb: add type parameter to value::allocate_register and add value::allocate_register_lazy

Some places that create register struct values don't use register_type
to obtain the value type.  This prevents them from using the current
version of value::allocate_register.  One spot (value_of_register_lazy)
also creates a lazy register value.

Add a value::allocate_register_lazy method.  Add some type parameters
to value::allocate_register and value::allocate_register_lazy, to let
the caller specify the type to use for the value.  The parameters
default to nullptr, in which case we use register_type to obtain the
type.

Change-Id: I640ec0a5a0f4a55eba12d515dbfd25933229f8ec