Process tilde in llvm::sys::path::native

Windows does not treat `~` as a reference to home directory, so the call
to `llvm::sys::path::native` on, say, `~/somedir` produces `~\somedir`,
which has different meaning than the original path. With this change
tilde is expanded on Windows to user profile directory. Such behavior
keeps original meaning of the path and is consistent with the algorithm
of `llvm::sys::path::home_directory`.

Differential Revision: https://reviews.llvm.org/D27527

llvm-svn: 296590