[clang][dataflow][NFC] Fix outdated comment on getStableStorageLocation

Follow-up to D129097.

It is no longer a requirement that the `QualType` passed to to
`DataflowAnalysisContext::getStableStorageLocation()` is not null. A
null type pass as an argument is only applicable as the pointee type
of a `std::nullptr_t` pointer.

Differential Revision: https://reviews.llvm.org/D131109

clang/include/clang/Analysis/FlowSensitive/DataflowAnalysisContext.h

@@ -93,9 +93,7 @@ public:
 
   /// Returns a new storage location appropriate for `Type`.
   ///
-  /// Requirements:
-  ///
-  ///  `Type` must not be null.
+  /// A null `Type` is interpreted as the pointee type of `std::nullptr_t`.
   StorageLocation &createStorageLocation(QualType Type);
 
   /// Returns a stable storage location for `D`.