lib: refactor unhandled rejection deprecation warning emission

Emit the deprecation warning in the `kDefaultUnhandledRejections`
case to reduce the number of branches on unhandled rejection mode -
there is now only one switch case on it.

Also rename `emitWarning()` to `emitUnhandledRejectionWarning()`
to avoid ambiguity with `process.emitWarning()`

lib/internal/process/promises.js

@@ -126,7 +126,7 @@ function handledRejection(promise) {
 }
 
 const unhandledRejectionErrName = 'UnhandledPromiseRejectionWarning';
-function emitWarning(uid, reason) {
+function emitUnhandledRejectionWarning(uid, reason) {
   const warning = getError(
     unhandledRejectionErrName,
     'Unhandled promise rejection. This error originated either by ' +
@@ -144,20 +144,15 @@ function emitWarning(uid, reason) {
   } catch {}
 
   process.emitWarning(warning);
-  emitDeprecationWarning();
 }
 
 let deprecationWarned = false;
 function emitDeprecationWarning() {
-  if (unhandledRejectionsMode === kDefaultUnhandledRejections &&
-      !deprecationWarned) {
-    deprecationWarned = true;
-    process.emitWarning(
-      'Unhandled promise rejections are deprecated. In the future, ' +
-      'promise rejections that are not handled will terminate the ' +
-      'Node.js process with a non-zero exit code.',
-      'DeprecationWarning', 'DEP0018');
-  }
+  process.emitWarning(
+    'Unhandled promise rejections are deprecated. In the future, ' +
+    'promise rejections that are not handled will terminate the ' +
+    'Node.js process with a non-zero exit code.',
+    'DeprecationWarning', 'DEP0018');
 }
 
 // If this method returns true, we've executed user code or triggered
@@ -186,7 +181,7 @@ function processPromiseRejections() {
       case kThrowUnhandledRejections: {
         fatalException(reason);
         const handled = process.emit('unhandledRejection', reason, promise);
-        if (!handled) emitWarning(uid, reason);
+        if (!handled) emitUnhandledRejectionWarning(uid, reason);
         break;
       }
       case kIgnoreUnhandledRejections: {
@@ -195,12 +190,16 @@ function processPromiseRejections() {
       }
       case kAlwaysWarnUnhandledRejections: {
         process.emit('unhandledRejection', reason, promise);
-        emitWarning(uid, reason);
+        emitUnhandledRejectionWarning(uid, reason);
         break;
       }
       case kDefaultUnhandledRejections: {
         const handled = process.emit('unhandledRejection', reason, promise);
-        if (!handled) emitWarning(uid, reason);
+        if (!handled) emitUnhandledRejectionWarning(uid, reason);
+        if (!deprecationWarned) {
+          emitDeprecationWarning();
+          deprecationWarned = true;
+        }
         break;
       }
     }