[clangd] Add -ffreestanding on VFS tests.

Summary:
They don't provide proper gcc installations and may fail on implicit
<stdc-predef.h> include.

Reviewers: klimek, krasimir, bkramer

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34936

llvm-svn: 307025