PTH: Hook up getSpelling() caching in PTHLexer. This results in a nice

performance gain. Here's what we see for -Eonly on Cocoa.h (using PTH):

- wall time decreases by 21% (26% speedup overall)
- system time decreases by 35%
- user time decreases by 6%

These reductions are due to not paging source files just to get spellings for
literals. The solution in place doesn't appear to be 100% yet, as we still see
some of the pages for source files getting mapped in. Using -print-stats, we see
that SourceManager maps in 7179K less bytes of source text (reduction of 75%).
Will investigate why the remaining 25% are getting paged in.

With these changes, here's how PTH compares to non-PTH on Cocoa.h:
  -Eonly: PTH takes 64% of the time as non-PTH (54% speedup)
  -fsyntax-only: PTH takes 89% of the time as non-PTH (11% speedup)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61913 91177308-0d34-0410-b5e6-96231b3b80d8

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