Split out filename & directory from DIFile to start generalizing over DIScopes

This is the first step to making all DIScopes have a common metadata prefix (so
that things (using directives, for example) that can appear in any scope can be
added to that common prefix). DIFile is itself a DIScope so the common prefix
of all DIScopes cannot be a DIFile - instead it's the raw filename/directory
name pair.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177239 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/DebugInfo.h

@@ -177,8 +177,6 @@ namespace llvm {
       if (DbgNode && !isFile())
         DbgNode = 0;
     }
-    StringRef getFilename() const  { return getStringField(1);   }
-    StringRef getDirectory() const { return getStringField(2);   }
     bool Verify() const;
   };