[-E] Emit a rewritten _Pragma on its own line.

1. Teach Lexer that pragma lexers are like macro expansions at EOF.
2. Treat pragmas like #define/#undef when printing.
3. If we just printed a directive, add a newline before any more tokens.
(4. Miscellaneous cleanup in PrintPreprocessedOutput.cpp)

PR10594 and <rdar://problem/11562490> (two separate related problems)

llvm-svn: 158571