[cmake] Use a sane default for LLVM_PROFILE_DATA_DIR
[cmake] Use a sane default for LLVM_PROFILE_DATA_DIR

It's been pointed out that arbitrarily spraying raw profiles into a
build directory is insane. Doing this wastes a tremendous amount of
space and is also very lossy, since the test harness tends to wipe away
temporary sub-directories (which usually contain relevant profile data).

The new default is a `profiles` directory inside of the build dir.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276504 91177308-0d34-0410-b5e6-96231b3b80d8

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