Implement libclang support for using declarations. Clang actually uses

three different kinds of AST nodes to represent using declarations:
UsingDecl, UnresolvedUsingValueDecl, and
UnresolvedUsingTypenameDecl. These three are collapsed into a single
cursor kind for using declarations, since libclang clients don't need
the distinction.

Several related changes here:
  - Cursor visitation of the three AST nodes for using declarations
  - Proper source-range computation for these AST nodes
  - Using declarations have no USRs, since they don't actually declare
    any entities.

llvm-svn: 112730