[clangd] Fixed a data race.

Summary:
Calling addDocument after removeDocument could have resulted in an
invalid program state (AST and Preamble for the valid document could
have been incorrectly removed).
This commit also includes an improved CppFile::cancelRebuild
implementation that allows to cancel reparse without waiting for
ongoing rebuild to finish.

Reviewers: krasimir, bkramer, klimek

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36397

llvm-svn: 310818