PGO instrumentation
    
    Add two PGO build conditions
    
    * pgo_instrumented_build
    * pgo_optimized_build
    
    Add `pgo-instrumentation-macros` subpackage.
    
    There were a couple off error that I ran into:
    
    `Error: LLVM Profile Warning: Unable to track new values: Running out of static counters. Consider using option -mllvm -vp-counters-per-site=<n> to allocate more value profile counters at compile time.`
    
    *Solution:* Add `--vp-counters-per-site` option
    
    Then a follow-up error occurred:
    
    `Error: clang (LLVM option parsing): for the --vp-counters-per-site option: may only occur zero or one times!`
    
    *Solution:* Modify `vp-counters-per-site` option through `LLVM_VP_COUNTERS_PER_SITE` instead of adding it.