Add nullptr check in maybeExtractNearestSourceLoc

For templated types which are pointers, check if the pointer is null in
maybeExtractNearestSourceLoc.

rdar://127251788

Author: augusto2112

include/swift/AST/SimpleRequest.h

@@ -100,6 +100,9 @@ template<typename T,
          typename = typename std::enable_if<
              canExtractNearestSourceLoc<T>()>::type>
 SourceLoc maybeExtractNearestSourceLoc(const T& value) {
+  if constexpr (std::is_pointer_v<T>)
+    if (value == nullptr)
+      return SourceLoc();
   return extractNearestSourceLoc(value);
 }
 

lib/AST/DeclContext.cpp

@@ -1418,6 +1418,8 @@ bool DeclContext::isAsyncContext() const {
 }
 
 SourceLoc swift::extractNearestSourceLoc(const DeclContext *dc) {
+  assert(dc && "Expected non-null DeclContext!");
+
   switch (dc->getContextKind()) {
   case DeclContextKind::Package:
   case DeclContextKind::Module: