Demangling: use assert over llvm_unreachable

Avoid a dependency on LLVMSupport at runtime through the `llvm_unreachable`.
This would pull in `llvm_unreachable_internal` in debug builds, which requires a
runtime dependency on LLVMSupport which increases the size of the binary
considerably.

Author: compnerd

lib/Demangling/Demangler.cpp

@@ -185,7 +185,7 @@ static bool isAliasNode(Demangle::NodePointer Node) {
   default:
     return false;
   }
-  llvm_unreachable("Unhandled node kind!");
+  assert(0 && "unknown node kind");
 }
 
 bool swift::Demangle::isAlias(llvm::StringRef mangledName) {
@@ -203,7 +203,7 @@ static bool isClassNode(Demangle::NodePointer Node) {
   default:
     return false;
   }
-  llvm_unreachable("Unhandled node kind!");
+  assert(0 && "unknown node kind");
 }
 
 bool swift::Demangle::isClass(llvm::StringRef mangledName) {
@@ -221,7 +221,7 @@ static bool isEnumNode(Demangle::NodePointer Node) {
   default:
     return false;
   }
-  llvm_unreachable("Unhandled node kind!");
+  assert(0 && "unknown node kind");
 }
 
 bool swift::Demangle::isEnum(llvm::StringRef mangledName) {
@@ -238,7 +238,7 @@ static bool isProtocolNode(Demangle::NodePointer Node) {
   default:
     return false;
   }
-  llvm_unreachable("Unhandled node kind!");
+  assert(0 && "unknown node kind");
 }
 
 bool swift::Demangle::isProtocol(llvm::StringRef mangledName) {
@@ -256,7 +256,7 @@ static bool isStructNode(Demangle::NodePointer Node) {
   default:
     return false;
   }
-  llvm_unreachable("Unhandled node kind!");
+  assert(0 && "unknown node kind");
 }
 
 bool swift::Demangle::isStruct(llvm::StringRef mangledName) {