compiler: make top-level decl for address-taken non-escaping locals

compiler: make top-level decl for address-taken non-escaping locals

    If a local variable's address is taken and passed out of its
    lexical scope, GCC backend may reuse the stack slot for the
    variable, not knowing it is still live through a pointer. In
    this case, we create a top-level temporary variable and let the
    user-defined variable refer to the temporary variable as its
    storage location. As the temporary variable is declared at the
    top level, its stack slot will remain live throughout the
    function.

    Reviewed-on: https://go-review.googlesource.com/84675

	* go-gcc.cc (local_variable): Add decl_var parameter.

From-SVN: r256398