[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/IDE/ModuleInterfacePrinting.cpp

@@ -53,8 +53,8 @@ public:
       ClangLoader(ClangLoader) {}
 
 private:
-  void printDeclPre(const Decl *D, Optional<BracketOptions> Bracket) override;
-  void printDeclPost(const Decl *D, Optional<BracketOptions> Bracket) override;
+  void printDeclPre(const Decl *D, llvm::Optional<BracketOptions> Bracket) override;
+  void printDeclPost(const Decl *D, llvm::Optional<BracketOptions> Bracket) override;
   void avoidPrintDeclPost(const Decl *D) override;
   // Forwarding implementations.
 
@@ -85,14 +85,14 @@ private:
   }
   void printSynthesizedExtensionPre(const ExtensionDecl *ED,
                                     TypeOrExtensionDecl Target,
-                                    Optional<BracketOptions> Bracket) override {
+                                    llvm::Optional<BracketOptions> Bracket) override {
     return OtherPrinter.printSynthesizedExtensionPre(ED, Target, Bracket);
   }
 
   void
   printSynthesizedExtensionPost(const ExtensionDecl *ED,
                                 TypeOrExtensionDecl Target,
-                                Optional<BracketOptions> Bracket) override {
+                                llvm::Optional<BracketOptions> Bracket) override {
     return OtherPrinter.printSynthesizedExtensionPost(ED, Target, Bracket);
   }
 
@@ -219,14 +219,14 @@ static bool extensionHasClangNode(ExtensionDecl *ext) {
   return static_cast<bool>(swift::ide::extensionGetClangNode(ext));
 }
 
-Optional<StringRef>
+llvm::Optional<StringRef>
 swift::ide::findGroupNameForUSR(ModuleDecl *M, StringRef USR) {
   for (auto File : M->getTopLevelModule()->getFiles()) {
     if (auto Name = File->getGroupNameByUSR(USR)) {
       return Name;
     }
   }
-  return None;
+  return llvm::None;
 }
 
 /// Prints a single decl using the \p Printer and \p Options provided. If
@@ -950,7 +950,7 @@ void ClangCommentPrinter::avoidPrintDeclPost(const Decl *D) {
 }
 
 void ClangCommentPrinter::printDeclPre(const Decl *D,
-                                       Optional<BracketOptions> Bracket) {
+                                       llvm::Optional<BracketOptions> Bracket) {
   // Skip parameters, since we do not gracefully handle nested declarations on a
   // single line.
   // FIXME: we should fix that, since it also affects struct members, etc.
@@ -968,7 +968,7 @@ void ClangCommentPrinter::printDeclPre(const Decl *D,
 }
 
 void ClangCommentPrinter::printDeclPost(const Decl *D,
-                                        Optional<BracketOptions> Bracket) {
+                                        llvm::Optional<BracketOptions> Bracket) {
   OtherPrinter.printDeclPost(D, Bracket);
 
   // Skip parameters; see printDeclPre().