[FileSystem] Add expand_tilde function

In D54435 there was some discussion about the expand_tilde flag for
real_path that I wanted to expose through the VFS. The consensus is that
these two things should be separate functions. Since we already have the
code for this I went ahead and added a function expand_tilde that does
just that.

Differential revision: https://reviews.llvm.org/D54448

llvm-svn: 346776