StringSwitch cannot be copied or moved.

...but most importantly, it cannot be used well with 'auto', because
the inferred type is StringSwitch rather than the result type. This
is a problem because StringSwitch stores addresses of temporary
values rather than copying or moving the value into its own storage.

Changing this uncovered the bug in PassBuilder, also in this patch.
Clang doesn't seem to have any occurrences of the issue.

llvm-svn: 276652

llvm/include/llvm/ADT/StringSwitch.h

@@ -53,6 +53,13 @@ public:
   explicit StringSwitch(StringRef S)
   : Str(S), Result(nullptr) { }
 
+  // StringSwitch is neither copyable nor movable.
+  StringSwitch(const StringSwitch &) = delete;
+  StringSwitch(StringSwitch &&) = delete;
+  void operator=(const StringSwitch &) = delete;
+  void operator=(StringSwitch &&) = delete;
+  ~StringSwitch() = default;
+
   template<unsigned N>
   LLVM_ATTRIBUTE_ALWAYS_INLINE
   StringSwitch& Case(const char (&S)[N], const T& Value) {

llvm/lib/Passes/PassBuilder.cpp

@@ -334,7 +334,7 @@ bool PassBuilder::parseModulePassName(ModulePassManager &MPM, StringRef Name,
       return false;
     assert(Matches.size() == 3 && "Must capture two matched strings!");
 
-    auto L = StringSwitch<OptimizationLevel>(Matches[2])
+    OptimizationLevel L = StringSwitch<OptimizationLevel>(Matches[2])
         .Case("O0", O0)
         .Case("O1", O1)
         .Case("O2", O2)