[clangd] Remove forceReparse, add a flag to addDocument instead

Summary: To make the removal of DraftMgr from ClangdServer easier (D44408).

Reviewers: sammccall, simark

Reviewed By: sammccall, simark

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

Differential Revision: https://reviews.llvm.org/D44462

llvm-svn: 327532