[CSSPGO] Fix an invalid hash table reference issue in the CS preinliner.

We were using a `StringMap` object to store all profiles to be emitted. The object is basically an unordered hash table, therefore updating it in the process of trasvering it may cause issue since the underlying bucket array could change.

I'm also moving the `csspgo-preinliner` switch around so that no context tri will be constructed (by the constructor of `CSPreInliner`) when the switch is off.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D104267