[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/Basic/StringExtras.cpp

@@ -21,6 +21,7 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Path.h"
@@ -30,6 +31,8 @@
 using namespace swift;
 using namespace camel_case;
 
+using llvm::StringRef;
+
 bool swift::canBeArgumentLabel(StringRef identifier) {
   return llvm::StringSwitch<bool>(identifier)
     .Case("inout", false)
@@ -344,8 +347,8 @@ size_t camel_case::findWord(StringRef string, StringRef word) {
 }
 
 /// Skip a type suffix that can be dropped.
-static Optional<StringRef> skipTypeSuffix(StringRef typeName) {
-  if (typeName.empty()) return None;
+static llvm::Optional<StringRef> skipTypeSuffix(StringRef typeName) {
+  if (typeName.empty()) return llvm::None;
 
   auto lastWord = camel_case::getLastWord(typeName);
 
@@ -381,7 +384,7 @@ static Optional<StringRef> skipTypeSuffix(StringRef typeName) {
   if (typeName.size() > 2 && typeName.endswith("_t")) {
     return typeName.drop_back(2);
   }
-  return None;
+  return llvm::None;
 }
 
 /// Match a word within a name to a word within a type.
@@ -718,7 +721,7 @@ omitSelfTypeFromBaseName(StringRef name, OmissionTypeName typeName,
   typeName.CollectionElement = StringRef();
 
   auto nameWords = camel_case::getWords(name);
-  Optional<llvm::iterator_range<WordIterator>> matchingRange;
+  llvm::Optional<llvm::iterator_range<WordIterator>> matchingRange;
 
   // Search backwards for the type name, whether anchored at the end or not.
   for (auto nameReverseIter = nameWords.rbegin();
@@ -1218,8 +1221,8 @@ bool swift::omitNeedlessWords(StringRef &baseName,
                               bool returnsSelf,
                               bool isProperty,
                               const InheritedNameSet *allPropertyNames,
-                              Optional<unsigned> completionHandlerIndex,
-                              Optional<StringRef> completionHandlerName,
+                              llvm::Optional<unsigned> completionHandlerIndex,
+                              llvm::Optional<StringRef> completionHandlerName,
                               StringScratchSpace &scratch) {
   bool anyChanges = false;
   OmissionTypeName resultType = returnsSelf ? contextType : givenResultType;
@@ -1367,7 +1370,7 @@ bool swift::omitNeedlessWords(StringRef &baseName,
   return lowercaseAcronymsForReturn();
 }
 
-Optional<StringRef> swift::stripWithCompletionHandlerSuffix(StringRef name) {
+llvm::Optional<StringRef> swift::stripWithCompletionHandlerSuffix(StringRef name) {
   if (name.endswith("WithCompletionHandler")) {
     return name.drop_back(strlen("WithCompletionHandler"));
   }
@@ -1392,7 +1395,7 @@ Optional<StringRef> swift::stripWithCompletionHandlerSuffix(StringRef name) {
     return name.drop_back(strlen("WithReply"));
   }
 
-  return None;
+  return llvm::None;
 }
 
 void swift::writeEscaped(llvm::StringRef Str, llvm::raw_ostream &OS) {