[clangd] Keep only a limited number of idle ASTs in memory

Summary:
After this commit, clangd will only keep the last 3 accessed ASTs in
memory. Preambles for each of the opened files are still kept in
memory to make completion and AST rebuilds fast.

AST rebuilds are usually fast enough, but having the last ASTs in
memory still considerably improves latency of operations like
findDefinition and documeneHighlight, which are often sent multiple
times a second when moving around the code. So keeping some of the last
accessed ASTs in memory seems like a reasonable tradeoff.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: malaperle, arphaman, klimek, javed.absar, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47063

llvm-svn: 333737