[CMAKE] Enable FatLTO as a build option for LLVM (#80480)

Since LLVM supports `-ffat-lto-objects` we should enable this as an
option in the LLVM build. FatLTO should improve the time it takes to
build tests for LTO enabled builds of the compiler by not linking w/ the
bitcode portion of the object files, which should speed up build times
for LTO builds without disabling optimizations.