[llvm][docs] Replace Optional<T> with std::optional<T>

The `llvm::Optional` template is no more. Remove it from the error-handling section of LLVM the programmer's manual and the ASTImporter documentation.

Author: jansvoboda11

clang/docs/LibASTImporter.rst

@@ -468,7 +468,7 @@ Note, there may be several different ASTImporter objects which import into the s
         cxxRecordDecl(hasName("Y"), isDefinition()), ToUnit);
     ToYDef->dump();
     // An error is set for "ToYDef" in the shared state.
-    Optional<ASTImportError> OptErr =
+    std::optional<ASTImportError> OptErr =
         ImporterState->getImportDeclErrorIfAny(ToYDef);
     assert(OptErr);
 

llvm/docs/ProgrammersManual.rst

@@ -590,14 +590,14 @@ semantics.  For example:
 
 This third form works with any type that can be assigned to from ``T&&``. This
 can be useful if the ``Expected<T>`` value needs to be stored an already-declared
-``Optional<T>``. For example:
+``std::optional<T>``. For example:
 
 .. code-block:: c++
 
   Expected<StringRef> extractClassName(StringRef Definition);
   struct ClassData {
     StringRef Definition;
-    Optional<StringRef> LazyName;
+    std::optional<StringRef> LazyName;
     ...
     Error initialize() {
       if (auto Err = extractClassName(Path).moveInto(LazyName))