gccrs: Remove backend dependancy on resolution rib information

gccrs: Remove backend dependancy on resolution rib information

When making more desugaring for the HIR we can need to add new Let bindings
which will require namesolution information but also rib information for
which block the let binding is associated which was very unnessecary. This
patch simply updates the BLOCK_CONTEXT of the current scope as we are
compiling and encounter HIR::LetStmts instead of trying to do it all
upfront which lots of wierd checks

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (HIRCompileBase::compile_locals_for_block): removed
	* backend/rust-compile-base.h: update header
	* backend/rust-compile-block.cc (CompileBlock::visit): remove old logic
	* backend/rust-compile-expr.cc (CompileExpr::generate_closure_function): likewise
	* backend/rust-compile-stmt.cc (CompileStmt::visit): likewise
	* backend/rust-compile-var-decl.h: ensure we setup tuple bindings correctly

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