Re-apply r296730, "cmake: Configure the ThinLTO cache directory when using ELF lld or gold."
Re-apply r296730, "cmake: Configure the ThinLTO cache directory when using ELF lld or gold."

All known cache-related bugs observed when self hosting have been fixed (r296907
and r297853).

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@297854 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm