[clangd] Rename ClangdUnit.h -> ParsedAST.h. NFC

This much better reflects what is (now) in this header.
Maybe a rename to ParsedTU would be an improvement, but that's a much
more invasive change and life is too short.

ClangdUnit is dead, long live ClangdUnitTests!

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@370862 91177308-0d34-0410-b5e6-96231b3b80d8
31 files changed