Enhance PTH 'getSpelling' caching:

- Refactor caching logic into a helper class PTHSpellingSearch
- Allow "random accesses" in the spelling cache, thus catching the remaining
  cases where 'getSpelling' wasn't hitting the PTH cache
  
For -Eonly, PTH, Cocoa.h:
- This reduces wall time by 3% (user time unchanged, sys time reduced)
- This reduces the amount of paged source by 1112K.
  The remaining 1112K still being paged in is from somewhere else
  (investigating).

llvm-svn: 62009