SourceManager: Implement "lazy" creation of MemBuffers for source files.

- Big Idea:
   Source files are now mmaped when ContentCache::getBuffer() is first called.
   While this doesn't change the functionality when lexing regular source files,
   it can result in source files not being paged in when using PTH.

- Performance change:
  - No observable difference (-fsyntax-only/-Eonly) on Cocoa.h when doing
    regular source lexing.
  - No observable time difference (-fsyntax-only/-Eonly) on Cocoa.h when using
    PTH. We do observe, however, a reduction of 279K in memory mapped source
    code (3% reduction). The majority of pages from Cocoa.h (and friends) are
    still being pulled in, however, because any literal will cause
    Preprocessor::getSpelling() to be called (causing the source for the file to
    get pulled in). The next possible optimization is to cache literal strings
    in the PTH file to avoid the need for the original header sources entirely.

- Right now there is a preprocessor directive to toggle between "lazy" and
  "eager" creation of MemBuffers. This is not permanent, and is there in the
  short term to just test additional optimizations.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61827 91177308-0d34-0410-b5e6-96231b3b80d8

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