[VFS] InMemoryFilesystem's UniqueIDs are a function of path and content.

This ensures that re-creating "the same" FS results in the same UIDs for files.
In turn, this means that creating a clang module (preamble) using one in-memory
filesystem and consuming it using another doesn't create duplicate FileEntrys
for files that are the same in both FSes.

It's tempting to give the creator control over the UIDs instead. However that
requires fiddly API changes, e.g. what should the UIDs of intermediate
directories be?
This change is more "magic" but seems safe given:
 - InMemoryFilesystem is used in testing more than production
 - comparing UIDs across filesystems is unusual
 - files with the same path and content are usually logically equivalent

(The usual reason for re-creating virtual filesystems rather than reusing them
is that typical use involves mutating their CWD and so is not threadsafe).

Differential Revision: https://reviews.llvm.org/D110711