[clangd] Simplify code handling compile commands

Summary:
CppFile can now change compilation arguments during rebuild. This allows
simplifying code that manages CppFiles.

Reviewers: sammccall, bkramer, jkorous-apple

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D42173

llvm-svn: 323204