emulators/wine-proton: Default to LLVM 15 (instead of 10 or 11)

Port dc3c11a from emulators/wine:

  If the user has set the default version of LLVM to 10 or 11, use
  LLVM 15 instead (rather than LLVM 12 as before). LLVM 15 is the
  current default in ports, so one less version to install in case.
  Plus it's what everyone else uses.

Approved by:	maintainer (iwtcex@gmail.com)

emulators/wine-proton/Makefile

@@ -106,7 +106,7 @@ PULSEAUDIO_CONFIGURE_OFF=	--without-pulse
 .include <bsd.port.pre.mk>
 
 .if (${LLVM_DEFAULT} == 10 || ${LLVM_DEFAULT} == 11)
-_LLVM_VERSION=	12
+_LLVM_VERSION=	15
 .else
 _LLVM_VERSION=	${LLVM_DEFAULT}
 .endif