[VFS] Add getter for mapping entries.
[VFS] Add getter for mapping entries.

When generating a reproducer in LLDB we build up the mapping but don't
immediately copy over the files on the file system.

Rather than keeping a separate data structure with real and virtual
paths, we might as well reuse the entries already stored in the
YAMLVFSWriter to lazily copy over the files when needed.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@351266 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit bf3266d130c74f715897dd860cec93ab42eed105)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm