Declare __fake_use_va_args as a constexpr function.

* Need a body and return value for this function
  so the constexpr check can inline it.
* C mode can just use an extern function declaration, but not constexpr.

Bug: 116854606
Bug: 111614304
Test: build with WITH_TIDY=1 and enable static analyzer checks
Change-Id: Ie3f4efbcabed99416d196b6c361a772b8c6a4035