63de20c0de
This fixes #139614 on non-clang compilers by moving `__has_warning` completely inside the `#if defined(__clang__)` block. This prevents a parse failure from compilers which don't recognize `__has_warning`. Original description: Followup to #138741. This adds the requested macro to silence `-Wunnecessary-virtual-specifier` when declaring virtual anchor functions in `final` classes, per [LLVM policy](https://llvm.org/docs/CodingStandards.html#provide-a-virtual-method-anchor-for-classes-in-headers). It also cleans up any remaining instances of the warning, allowing us to stop disabling it when we build LLVM.