[VirtualFileSystem] InMemoryFileSystem::addFile(): Type and Perms

Summary:
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)

Reviewers: bkramer, hokein

Reviewed By: bkramer, hokein

Subscribers: alexfh

Differential Revision: https://reviews.llvm.org/D39572

llvm-svn: 317800