Basic: adjust aliasing annotation (NFCI)

GCC `-fpermissive` flagged this annotation as being discarded as it is
being applied after the definition.  Move the annotation to the
re-definition to which it appertains.

Author: compnerd

lib/Basic/PrefixMap.cpp

@@ -34,8 +34,8 @@ enum class ChildKind { Left, Right, Further, Root };
 // that's technically instantiation-specific.  Redefining the struct here
 // is technically an aliasing violation, but we can just tell the compilers
 // that actually use TBAA that this is okay.
-typedef struct _Node Node LLVM_MAY_ALIAS;
-struct _Node {
+typedef struct _Node Node;
+struct LLVM_MAY_ALIAS _Node {
   // If you change the layout in the header, you'll need to change it here.
   // (This comment is repeated there.)
   Node *Left, *Right, *Further;