[clangd] Added a test for preambles and -isystem
[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

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@336530 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra