Renaming header guards to match the LLVM convention.
This patch was created by automatically applying the fixes from
clang-tidy.
I've removed the [NFC] tag from the title, as we're adding header guards in some files and thus might trigger behavior changes.
Differential Revision: https://reviews.llvm.org/D113896
TestWorkspace allows easily writing tests involving multiple
files that can have inclusion relationships between them.
BackgroundIndexTest.RelationsMultiFile is refactored to use
TestWorkspace, and moved to FileIndexTest as it no longer
depends on BackgroundIndex.
Differential Revision: https://reviews.llvm.org/D89297