[llvm] Fix small typos in Programmer's Manual and Chrono.h.

The Programmer's Manual guidance on the StringRef class (https://llvm.org/docs/ProgrammersManual.html#the-stringref-class) refers to `iterator StringRef::find(StringRef Key)` which does not exist. Based on context this is surely a small typo meant to be `iterator StringMap::find(StringRef Key)`.

This also corrects some small typos in the comments of llvm/include/llvm/Support/Chrono.h

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D113462