emulators/wine: Firmly depend on LLVM 15

Our devel/llvm18 and devel/llvm17 ports (with commits c56fde6514 and
b21e6b4de1, respectively) now provide C99 include files.[1] This is
a positive and fixes a long standing issue. It significantly changes
what Wine builds, though. For emulators/wine-devel we moved to the new
world order already; for emulators/wine remain a bit more conservative
and stay with status quo ante by firmly using LLVM 15.

For most users this won't make any difference since so far we have
been using LLVM_DEFAULT which currently stands at ... 15.

On the way use the new USES=llvm facility instead of doing things
more manually.

PR: 279677, 274542 [1]