Don't read all of the records in the PCH file's preprocessor block,

most of which are ignored. Instead, move the __COUNTER__ value out to
a PCH-level record (since it is handled eagerly) and move the header
file information into the SourceManager block (which is also,
currently, loaded eagerly).

This results in another 17% performance improvement in the
Cocoa-prefixed "Hello, World" with PCH.

llvm-svn: 70097