[clangd] Added a test for preambles and -isystem

Summary:
Checks that preambles are properly invalidated when headers from
-isystem paths change.

Reviewers: sammccall, ioeric

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48947

llvm-svn: 336530