[Tooling] JSONCompilationDatabasePlugin infers compile commands for missing files

Summary:
See the existing InterpolatingCompilationDatabase for details on how this works.
We've been using this in clangd for a while, the heuristics seem to work well.

Reviewers: bkramer

Subscribers: ilya-biryukov, ioeric, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51729

llvm-svn: 342228