Add some llvm_unreachable annotations for recently introduced MSVC control path warnings

Author: hughbe

include/swift/SIL/SILFunctionConventions.h

@@ -357,6 +357,8 @@ inline bool SILModuleConventions::isIndirectSILResult(SILResultInfo result,
   case ResultConvention::Autoreleased:
     return false;
   }
+
+  llvm_unreachable("Unhandled ResultConvention in switch.");
 }
 
 inline SILType SILModuleConventions::getSILParamType(SILParameterInfo param,

lib/IDE/CodeCompletion.cpp

@@ -1354,6 +1354,8 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
     case DeclContextKind::TopLevelCodeDecl:
       return typeCheckTopLevelCodeDecl(cast<TopLevelCodeDecl>(DC));
     }
+
+    llvm_unreachable("Unhandled DeclContextKind in switch.");
   }
 
   /// \returns true on success, false on failure.

lib/SILGen/Cleanup.cpp

@@ -245,6 +245,8 @@ llvm::raw_ostream &Lowering::operator<<(llvm::raw_ostream &os,
   case CleanupState::PersistentlyActive:
     return os << "PersistentlyActive";
   }
+
+  llvm_unreachable("Unhandled CleanupState in switch.");
 }
 
 void CleanupManager::dump() const {