[clangd] Avoid memory-mapping files on Windows

Summary:
Memory-mapping files on Windows leads to them being locked and prevents
editors from saving changes to those files on disk. This is fine for the
compiler, but not acceptable for an interactive tool like clangd.
Therefore, we choose to avoid using memory-mapped files on Windows.

Reviewers: hokein, kadircet

Reviewed By: kadircet

Subscribers: yvvan, zturner, nik, malaperle, mgorny, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D55139

llvm-svn: 348147