Refine SourceManager's isBeforeInTranslationUnit() cache to have more entries.

isBeforeInTranslationUnit() uses a cache to reduce the expensive work
to compute a common ancestor for two FileIDs.  This work is very
expensive, so even caching the latest used FileIDs was a big win.
A closer analysis of the cache before, however, shows that the cache
access pattern would oscillate between a working set of FileIDs, and
thus caching more pairs would be profitable.

This patch adds a side table for extending caching.  This side table
is bounded in size (experimentally determined in this case from
a simple Objective-C project), and when the table gets too large
we fall back to the single entry caching before as before.

On Sketch (a small example Objective-C project), this optimization
reduces -fsyntax-only time on SKTGraphicView.m by 5%.  This is
for a project that is already using PCH.

Fixes <rdar://problem/13299847>

llvm-svn: 176142