Add a cache to SourceManager to accellerate line # lookup. This is a

bottleneck for -E computation, because every token that starts a line needs
to determine *which* line it is on (so -E mode can insert the appropriate 
vertical whitespace).  This optimization improves this common case where
it is striding through the line # table.

This speeds up -E on xalancbmk by 3.2%

llvm-svn: 40459