[MemProf] Control availability of hot/cold operator new from LTO link

Adds an LTO option to indicate that whether we are linking with an
allocator that supports hot/cold operator new interfaces. If not,
at the start of the LTO backends any existing memprof hot/cold
attributes are removed from the IR, and we also remove memprof metadata
so that post-LTO inlining doesn't add any new attributes.

This is done via setting a new flag in the module summary index. It is
important to communicate via the index to the LTO backends so that
distributed ThinLTO handles this correctly, as they are invoked by
separate clang processes and the combined index is how we communicate
information from the LTO link. Specifically, for distributed ThinLTO the
LTO related processes look like:
```
   # Thin link:
   $ lld --thinlto-index-only obj1.o ... objN.o -llib ...
   # ThinLTO backends:
   $ clang -x ir obj1.o -fthinlto-index=obj1.o.thinlto.bc -c -O2
   ...
   $ clang -x ir objN.o -fthinlto-index=objN.o.thinlto.bc -c -O2
```
...