Parse compile commands lazily in InterpolatingCompilationDatabase

Summary:
This greatly reduces the time to read 'compile_commands.json'.
For Chromium on my machine it's now 0.7 seconds vs 30 seconds before the
change.

Reviewers: sammccall, jfb

Reviewed By: sammccall

Subscribers: mgrang, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D51314

llvm-svn: 340838