Fix misuses of llvm::StringSwitch.

StringSwitch forms references to temporaries, so it cannot be stored
into a local. (Really 'auto' is to blame here; see LLVM r276671.)

Author: jrose-apple

lib/IRGen/GenBuiltin.cpp

@@ -487,7 +487,7 @@ if (Builtin.ID == BuiltinValueKind::id) { \
     auto underscore = BuiltinName.find('_');
     StringRef SubOp = BuiltinName.substr(0, underscore);
 
-    auto SubOpcode = StringSwitch<AtomicRMWInst::BinOp>(SubOp)
+    AtomicRMWInst::BinOp SubOpcode = StringSwitch<AtomicRMWInst::BinOp>(SubOp)
       .Case("xchg", AtomicRMWInst::Xchg)
       .Case("add",  AtomicRMWInst::Add)
       .Case("sub",  AtomicRMWInst::Sub)

lib/SILOptimizer/PassManager/PassManager.cpp

@@ -664,7 +664,7 @@ void SILPassManager::addPass(PassKind Kind) {
 }
 
 void SILPassManager::addPassForName(StringRef Name) {
-  auto P = llvm::StringSwitch<PassKind>(Name)
+  PassKind P = llvm::StringSwitch<PassKind>(Name)
 #define PASS(ID, NAME, DESCRIPTION) .Case(#ID, PassKind::ID)
 #include "swift/SILOptimizer/PassManager/Passes.def"
   ;