[Docs] Increase Doxygen cache size

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.

Reviewed By: hintonda

Patch by J. Ryan Stinnett!

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D62138

llvm-svn: 361343