[clangd] Use Contents from inputs in codeComplete and signatureHelp

Summary:
ClangdServer::{codeComplete,signatureHelp} both use the Contents from
the draft manager.  Since we want to move the draft manager from
ClangdServer to ClangdLSPServer, this patch changes those methods to
find the file contents from InputsAndPreamble, which contains the source
passed in previously.

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D44484

llvm-svn: 327550