[Docs] Increase Doxygen cache size
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
Reviewed By: hintonda
Patch by J. Ryan Stinnett!
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D62138