PreprocessorLexer (and subclasses):

- Added virtual method 'getSourceLocation()' (no arguments) that gets the location of the next "observable" location (e.g., next character, next token).
PPLexerChange.cpp:
- Implemented FIXME by using PreprocessorLexer::getSourceLocation() to get the location in the file we are returning to after lexing a #included file.  This appears to be slightly faster than having the branch (i.e., 'if(CurLexer)').  It's also not a really hot part of the Preprocessor.

llvm-svn: 60860