[clangd] Move getBeginningOfIdentifier from ClangdUnit to SourceCode. Drop dependencies on ClangdUnit from some headers. NFC

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@370768 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed