[Docs] Increase Doxygen cache size
[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

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@361343 91177308-0d34-0410-b5e6-96231b3b80d8

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