[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.

llvm-svn: 276504