[NFC] add llvm namespace to Optional and None

This is phase-1 of switching from llvm::Optional to std::optional in the
next rebranch. llvm::Optional was removed from upstream LLVM, so we need
to migrate off rather soon. On Darwin, std::optional, and llvm::Optional
have the same layout, so we don't need to be as concerned about ABI
beyond the name mangling. `llvm::Optional` is only returned from one
function in
```
getStandardTypeSubst(StringRef TypeName,
                     bool allowConcurrencyManglings);
```
It's the return value, so it should not impact the mangling of the
function, and the layout is the same as `std::optional`, so it should be
mostly okay. This function doesn't appear to have users, and the ABI was
already broken 2 years ago for concurrency and no one seemed to notice
so this should be "okay".

I'm doing the migration incrementally so that folks working on main can
cherry-pick back to the release/5.9 branch. Once 5.9 is done and locked
away, then we can go through and finish the replacement. Since `None`
and `Optional` show up in contexts where they are not `llvm::None` and
`llvm::Optional`, I'm preparing the work now by going through and
removing the namespace unwrapping and making the `llvm` namespace
explicit. This should make it fairly mechanical to go through and
replace llvm::Optional with std::optional, and llvm::None with
std::nullopt. It's also a change that can be brought onto the
release/5.9 with minimal impact. This should be an NFC change.

lib/DependencyScan/ModuleDependencyCacheSerialization.cpp

@@ -252,7 +252,7 @@ bool ModuleDependenciesCacheDeserializer::readGraph(SwiftDependencyScanningServi
       auto outputModulePath = getIdentifier(outputPathFileID);
       if (!outputModulePath)
          llvm::report_fatal_error("Bad .swiftmodule output path");
-      Optional<std::string> optionalSwiftInterfaceFile;
+      llvm::Optional<std::string> optionalSwiftInterfaceFile;
       if (interfaceFileID != 0) {
         auto swiftInterfaceFile = getIdentifier(interfaceFileID);
         if (!swiftInterfaceFile)
@@ -622,7 +622,7 @@ llvm::Optional<std::string> ModuleDependenciesCacheDeserializer::getIdentifier(u
 
   --n;
   if (n >= Identifiers.size())
-    return None;
+    return llvm::None;
 
   return Identifiers[n];
 }
@@ -633,7 +633,7 @@ llvm::Optional<std::vector<std::string>> ModuleDependenciesCacheDeserializer::ge
 
   --n;
   if (n >= ArraysOfIdentifierIDs.size())
-    return None;
+    return llvm::None;
 
   auto &identifierIDs = ArraysOfIdentifierIDs[n];
 
@@ -678,7 +678,7 @@ llvm::Optional<std::vector<ModuleDependencyID>> ModuleDependenciesCacheDeseriali
     return result;
   }
 
-  return None;
+  return llvm::None;
 }
 
 bool swift::dependencies::module_dependency_cache_serialization::
@@ -805,7 +805,7 @@ class ModuleDependenciesCacheSerializer {
   void writeArraysOfIdentifiers();
 
   void writeModuleInfo(ModuleDependencyID moduleID,
-                       Optional<std::string> contextHash,
+                       llvm::Optional<std::string> contextHash,
                        const ModuleDependencyInfo &dependencyInfo);
 
 public:
@@ -895,7 +895,7 @@ void ModuleDependenciesCacheSerializer::writeArraysOfIdentifiers() {
 }
 
 void ModuleDependenciesCacheSerializer::writeModuleInfo(ModuleDependencyID moduleID,
-                                 Optional<std::string> contextHash,
+                                 llvm::Optional<std::string> contextHash,
                                  const ModuleDependencyInfo &dependencyInfo) {
   using namespace graph_block;
   auto contextHashStrID = contextHash.has_value() ? getIdentifier(contextHash.value()) : 0;