[clangd] Moved caching of compile commands to ClangdServer

Summary:
It allows to get rid of CppFile::getLastCommand and simplify the
code in the upcoming threading patch.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42429

llvm-svn: 323420