Introduce ScopedDecl::getLexicalDeclContext() which is different from...

Introduce ScopedDecl::getLexicalDeclContext() which is different from ScopedDecl::getDeclContext() when there are nested-names.
e.g.:
  namespace A {
    void f(); // SemanticDC (getDeclContext) == LexicalDC (getLexicalDeclContext) == 'namespace A'
  }
  void A::f(); // SemanticDC == namespace 'A'
               // LexicalDC == global namespace

llvm-svn: 58948