[Docs] Increase Doxygen cache size

[jryans]

Summary

When building Doxygen docs for llvm and clang, it helpfully prints a warning at
the end noting that the LOOKUP_CACHE_SIZE value was too small to keep all
symbols in memory.

By increasing to the size it recommends, Doxygen builds have greatly improved
performance. On my machine, time to run doxygen-llvm changes from 34 minutes
to 22 minutes, which is a decent amount of time saved by changing a single
number.

[jryans]

I don't currently have commit access, so I would need someone to land this for me assuming it's approved. If it would be easier for me to submit separate patches for the llvm and clang changes, please let me know! 😄

[hintonda]

Thanks, LGTM!

I'll give it a day for further comments, and barring any, commit it for you tomorrow.

docs/doxygen.cfg.in

 # code, doxygen keeps a cache of pre-resolved symbols. If the cache is too small
 # doxygen will become slower. If the cache is too large, memory is wasted. The
 # cache size is given by this formula: 2^(16+LOOKUP_CACHE_SIZE). The valid range
 # is 0..9, the default is 0, corresponding to a cache size of 2^16=65536
 # symbols. At the end of a run doxygen will report the cache usage and suggest
 # the optimal cache size from a speed point of view.
 # Minimum value: 0, maximum value: 9, default value: 0.
 
-LOOKUP_CACHE_SIZE      = 2
+LOOKUP_CACHE_SIZE      = 3
 
 #---------------------------------------------------------------------------
 # Build related configuration options
 #---------------------------------------------------------------------------
 
 # If the EXTRACT_ALL tag is set to YES doxygen will assume all entities in
 # documentation are documented, even if no documentation was available. Private
 # class members and static file members will be hidden unless the