This implements a FIXME in InMemoryFileSystem::addFile(), allowing
clients to specify User, Group, Type, and/or Perms when creating a
file in an in-memory filesystem.
New tests included. Ran tests with:
% ninja BasicTests && ./tools/clang/unittests/Basic/BasicTests
Fixes PR#35172 (https://bugs.llvm.org/show_bug.cgi?id=35172)