NFC: Use VersionRange::all() to represent "always available".

It doesn't make sense to use `VersionRange::empty()` to represent "universally
available" since something that is available in an empty version range is
effectively never available.

Author: tshortli

lib/IRGen/GenMeta.cpp

@@ -2535,7 +2535,7 @@ namespace {
 
           SmallVector<llvm::BasicBlock *, 4> conditionalTypes;
 
-          // Pre-allocate a basic block per condition, so there it's
+          // Pre-allocate a basic block per condition, so that it's
           // possible to jump between conditions.
           for (unsigned index : indices(substitutionSet)) {
             conditionalTypes.push_back(
@@ -2619,7 +2619,7 @@ namespace {
           auto universal = substitutionSet.back();
 
           assert(universal->getAvailability().size() == 1 &&
-                 universal->getAvailability()[0].first.isEmpty());
+                 universal->getAvailability()[0].first.isAll());
 
           IGF.Builder.CreateRet(
               getResultValue(IGF, genericEnv, universal->getSubstitutions()));

lib/Sema/MiscDiagnostics.cpp

@@ -3631,7 +3631,7 @@ class OpaqueUnderlyingTypeChecker : public ASTWalker {
     // Add universally available choice as the last one.
     conditionalSubstitutions.push_back(
         OpaqueTypeDecl::ConditionallyAvailableSubstitutions::get(
-            Ctx, {{VersionRange::empty(), /*unavailable=*/false}},
+            Ctx, {{VersionRange::all(), /*unavailable=*/false}},
             std::get<1>(universallyAvailable)
                 .mapReplacementTypesOutOfContext()));
 

lib/Serialization/Deserialization.cpp

@@ -4559,7 +4559,7 @@ class DeclDeserializer {
         } else {
           limitedAvailability.push_back(
               OpaqueTypeDecl::ConditionallyAvailableSubstitutions::get(
-                  ctx, {{VersionRange::empty(), /*unavailability=*/false}},
+                  ctx, {{VersionRange::all(), /*unavailability=*/false}},
                   subMapOrError.get()));
 
           opaqueDecl->setConditionallyAvailableSubstitutions(limitedAvailability);