Add a "TypeSpecStartLoc" to FieldDecl. Patch contributed by Enea Zaffanella.

Note: One day, it might be useful to consider adding this info to DeclGroup (as the comments in FunctionDecl/VarDecl suggest). For now, I think this works fine. I considered moving this to ValueDecl (a common ancestor of FunctionDecl/VarDecl/FieldDecl), however this would add overhead to EnumConstantDecl (which would burn memory and isn't necessary).
llvm-svn: 75635