PTH: Use Token::setLiteralData() to directly store a pointer to cached spelling data...

PTH: Use Token::setLiteralData() to directly store a pointer to cached spelling data in the PTH file.  This removes a ton of code for looking up spellings using sourcelocations in the PTH file.  This simplifies both PTH-generation and reading.

Performance impact for -fsyntax-only on Cocoa.h (with Cocoa.h in the PTH file):
- PTH generation time improves by 5%
- PTH reading improves by 0.3%.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63072 91177308-0d34-0410-b5e6-96231b3b80d8