gccrs: Refactor query_type to only fully-const resolve const fns

When we are in a const context what can happen is we start to query all
non const types during method resolution for example which means its going
to query fn signitures and fully resolve their blocks for non const
candidates.

gcc/rust/ChangeLog:

	* typecheck/rust-type-util.cc (query_type): dont query non const candidates

Signed-off-by: Philip Herron <herron.philip@googlemail.com>