[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: https://llvm.org/svn/llvm-project/llvm/trunk@351266 91177308-0d34-0410-b5e6-96231b3b80d8

Author: JDevlieghere

include/llvm/Support/VirtualFileSystem.h

@@ -753,6 +753,8 @@ class YAMLVFSWriter {
     OverlayDir.assign(OverlayDirectory.str());
   }
 
+  const std::vector<YAMLVFSEntry> &getMappings() const { return Mappings; }
+
   void write(llvm::raw_ostream &OS);
 };