[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@361343 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm