AST: disambiguate swift::detail namespace (NFC)

swift::detail is ambiguous when building lldb for MSVC.  It is unable to
disambiguate between `lldb::swift::detail` and `swift::detail`.
Explicitly qualify the namespace to help it disambiguate it.

Author: compnerd

include/swift/AST/ClangNode.h

@@ -111,7 +111,7 @@ class ClangNode {
 namespace llvm {
 template <typename T>
 struct PointerLikeTypeTraits<swift::detail::ClangNodeBox<T>> {
-  using Box = swift::detail::ClangNodeBox<T>;
+  using Box = ::swift::detail::ClangNodeBox<T>;
 
   static inline void *getAsVoidPointer(Box P) {
     return const_cast<void *>(static_cast<const void *>(P.value));