[Frontend] Removed dead case.

It is no longer possible to have no lexical markers.  Remove the
corresponding case.

Author: nate-chandler

include/swift/AST/SILOptions.h

@@ -31,9 +31,6 @@
 namespace swift {
 
 enum class LexicalLifetimesOption : uint8_t {
-  // Do not insert lexical markers.
-  Off = 0,
-
   // Insert lexical markers via lexical borrow scopes and the lexical flag on
   // alloc_stacks produced from alloc_boxes, but strip them when lowering out of
   // Raw SIL.

include/swift/SIL/SILModule.h

@@ -1075,10 +1075,9 @@ inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SILModule &M){
 inline bool SILOptions::supportsLexicalLifetimes(const SILModule &mod) const {
   switch (mod.getStage()) {
   case SILStage::Raw:
-    // In raw SIL, lexical markers are used for diagnostics.  These markers are
-    // present as long as the lexical lifetimes feature is not disabled
-    // entirely.
-    return LexicalLifetimes != LexicalLifetimesOption::Off;
+    // In raw SIL, lexical markers are used for diagnostics and are always
+    // present.
+    return true;
   case SILStage::Canonical:
   case SILStage::Lowered:
     // In Canonical SIL, lexical markers are used to ensure that object

lib/AST/ASTContext.cpp

@@ -6532,5 +6532,5 @@ Type ASTContext::getNamedSwiftType(ModuleDecl *module, StringRef name) {
 
 bool ASTContext::supportsMoveOnlyTypes() const {
   // currently the only thing holding back whether the types can appear is this.
-  return SILOpts.LexicalLifetimes != LexicalLifetimesOption::Off;
+  return true;
 }