[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.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -244,7 +244,7 @@ struct ExplicitSwiftModuleInputInfo {
                                llvm::Optional<std::vector<std::string>> headerDependencyPaths,
                                bool isFramework = false,
                                bool isSystem = false,
-                               llvm::Optional<std::string> moduleCacheKey = None)
+                               llvm::Optional<std::string> moduleCacheKey = llvm::None)
     : modulePath(modulePath),
       moduleDocPath(moduleDocPath),
       moduleSourceInfoPath(moduleSourceInfoPath),
@@ -274,7 +274,7 @@ struct ExplicitClangModuleInputInfo {
                                std::string modulePath,
                                bool isFramework = false,
                                bool isSystem = false,
-                               llvm::Optional<std::string> moduleCacheKey = None)
+                               llvm::Optional<std::string> moduleCacheKey = llvm::None)
     : moduleMapPath(moduleMapPath),
       modulePath(modulePath),
       isFramework(isFramework),