[clang] Add flag to experiment with cold function attributes (#89298)

To be removed and promoted to a proper driver flag if experiments turn
out fruitful.

For now, this can be experimented with `-mllvm
-pgo-cold-func-opt=[optsize|minsize|optnone|default] -mllvm
-enable-pgo-force-function-attrs`.

Original LLVM patch for this functionality: #69030