AST: Avoid invoking TypeAliasDecl::getUnderlyingType() from isInterestingTypealias()

Diagnostics are emitted from weird places sometimes, and as a result the emission
is fragile for two reasons:

- Kicking off requests can trigger cycles
- Doing anything that can re-entrantly diagnose something else might crash

In this case, the problem is getUnderlyingType(), which might kick off type
resolution on the typealias. Since we already have a Type, just check that
instead of getting a new one.

Author: slavapestov

lib/AST/DiagnosticEngine.cpp

@@ -480,7 +480,7 @@ static bool isInterestingTypealias(Type type) {
   else
     return false;
 
-  if (aliasDecl->getUnderlyingType()->isVoid())
+  if (type->isVoid())
     return false;
 
   // The 'Swift.AnyObject' typealias is not 'interesting'.