[cmake] Allow LLVM_BUILD_INSTRUMENTED to be set to IR or Frontend

- This deprecates LLVM_ENABLE_IR_PGO but keeps it around for now.
- Errors out when LLVM_BUILD_INSTRUMENTED and LLVM_BUILD_INSTRUMENTED_COVERAGE
  are both set.

Motivated by bogner's post-commit review of r313770.

llvm-svn: 317725