qt5.qtwebengine: pin clang version to clang 15

The version of Chromium used by Qt WebEngine 5.15.x does not build with
clang 16 due to the following errors:

* -Wenum-constexpr-conversion: This is a downgradable error in clang 16,
  but it is planned to be made into a hard error in a future version of
  clang. Patches are not available for the version of v8 used by
  Chromium in Qt WebEngine, and fixing the code is non-trivial.
* -Wincompatible-function-pointer-types: This is also a downgradable
  error generated starting with clang 16. Patches are available upstream
  that can be backported.

Because the first error is non-trivial to fix and suppressing it risks
future breakage, clang is pinned to clang 15, which also makes fixing
the incompatible function pointer conversion errors unnecessary.

The derivation is also updated to use the `overrideLibcxx` adapter.
Using it links qt5.qtwebengine against the same versions of libc++ and
libc++abi as qt5.qtbase even though they are built with different
versions of clang, avoiding the compatibility problems that can cause.

Also, reenable x86_64-darwin because it builds successfully too.

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