gccrs: resolver: Refactor assertion

gccrs: resolver: Refactor assertion

Change the condition with rust_unreachable to an assertion. This will
keep the code clean and concise.
Some styling issues appeared during review, this commit make the code
more readable.

gcc/rust/ChangeLog:

	* resolve/rust-toplevel-name-resolver-2.0.cc (TopLevel::visit):
	Change to assertion.
	* util/rust-hir-map.cc (Mappings::lookup_derive_proc_macros):
	Add empty line.
	(Mappings::lookup_bang_proc_macros): Likewise.
	(Mappings::lookup_attribute_proc_macros): Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>