[ADT] Deprecate MutableArrayRef(std::nullopt) (#146113)

ArrayRef(std::nullopt) just got deprecated.  This patch does the same
to MutableArrayRef(std::nullopt).  Since there are only a couple of
uses, this patch does migration and deprecation at the same time.

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -302,7 +302,7 @@ namespace clang {
     }
     MutableArrayRef<FunctionTemplateSpecializationInfo>
     getPartialSpecializations(FunctionTemplateDecl::Common *) {
-      return std::nullopt;
+      return {};
     }
 
     template<typename DeclTy>

llvm/include/llvm/ADT/ArrayRef.h

@@ -304,7 +304,8 @@ namespace llvm {
     /*implicit*/ MutableArrayRef() = default;
 
     /// Construct an empty MutableArrayRef from std::nullopt.
-    /*implicit*/ MutableArrayRef(std::nullopt_t) : ArrayRef<T>() {}
+    /*implicit*/ LLVM_DEPRECATED("Use {} or MutableArrayRef<T>() instead", "{}")
+    MutableArrayRef(std::nullopt_t) : ArrayRef<T>() {}
 
     /// Construct a MutableArrayRef from a single element.
     /*implicit*/ MutableArrayRef(T &OneElt) : ArrayRef<T>(OneElt) {}

mlir/lib/IR/Diagnostics.cpp

@@ -727,7 +727,7 @@ SourceMgrDiagnosticVerifierHandlerImpl::computeExpectedDiags(
     raw_ostream &os, llvm::SourceMgr &mgr, const llvm::MemoryBuffer *buf) {
   // If the buffer is invalid, return an empty list.
   if (!buf)
-    return std::nullopt;
+    return {};
   auto &expectedDiags = expectedDiagsPerFile[buf->getBufferIdentifier()];
 
   // The number of the last line that did not correlate to a designator.