gccrs: Implement let statement support for IdentifierPattern's subpatterns

gccrs: Implement let statement support for IdentifierPattern's subpatterns

Trimmed GIMPLE code gen for let-identifierpattern-subpattern.rs

...
      RUSTTMP.1.__0 = 0;
      RUSTTMP.1.__1 = 2;
      RUSTTMP.1.__2 = 3;
      bar = RUSTTMP.1.__0;
      RUSTTMP.2 = RUSTTMP.1.__1;
      RUSTTMP.3 = RUSTTMP.1.__2;
      foo.__0 = 0;
      foo.__1 = 2;
      foo.__2 = 3;
      ret = 1;
      RUSTTMP.5 = foo;
      _1 = RUSTTMP.5.__0;
      _2 = _1 == 0;
      _3 = RUSTTMP.5.__1;
      _4 = _3 == 2;
      _5 = _2 & _4;
      _6 = RUSTTMP.5.__2;
      _7 = _6 == 3;
      _8 = _5 & _7;
      if (_8 != 0) goto <D.143>; else goto <D.144>;
      <D.143>:
      {
        {
          ret = bar;
        }
        goto <D.137>;
      }
...

gcc/rust/ChangeLog:

	* backend/rust-compile-pattern.cc(CompilePatternLet::visit(IdentifierPattern)):
	Add support for subpatterns.
	* backend/rust-compile-var-decl.h(CompileVarDecl::visit(IdentifierPattern)):
	Implement compilation for subpatterns.

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