gccrs: Implement missing var decl case for TuplePatternItemsHasRest

gccrs: Implement missing var decl case for TuplePatternItemsHasRest

GIMPLE dump from compiling issue-3930.rs:

...
  const i32 b;
  const i32 a;

  D.114.__0 = 2;
  D.114.__1 = 3;
  RUSTTMP.1 = D.114;
  a = RUSTTMP.1.__0;
  b = RUSTTMP.1.__1;
...

gcc/rust/ChangeLog:

	* backend/rust-compile-var-decl.h (CompileVarDecl::visit(TuplePattern)): Implement
	variable declaration bindings for tuple patterns with rest pattern (i.e.
	TuplePatternItemsHasRest).

Signed-off-by: Yap Zhi Heng <yapzhhg@gmail.com>