[CodeCompletion] Add defensive nullptr check

Prevent crash.
rdar://problem/51599533

Author: rintaro

lib/IDE/ExprContextAnalysis.cpp

@@ -367,15 +367,15 @@ static bool collectPossibleCalleesForApply(
 
   if (auto *DRE = dyn_cast<DeclRefExpr>(fnExpr)) {
     if (auto *decl = DRE->getDecl()) {
-      auto declType = decl->getInterfaceType();
-      if (auto *funcType = declType->getAs<AnyFunctionType>())
-        candidates.emplace_back(funcType, decl);
+      if (decl->hasInterfaceType())
+        if (auto *funcType = decl->getInterfaceType()->getAs<AnyFunctionType>())
+          candidates.emplace_back(funcType, decl);
     }
   } else if (auto *OSRE = dyn_cast<OverloadSetRefExpr>(fnExpr)) {
     for (auto *decl : OSRE->getDecls()) {
-      auto declType = decl->getInterfaceType();
-      if (auto *funcType = declType->getAs<AnyFunctionType>())
-        candidates.emplace_back(funcType, decl);
+      if (decl->hasInterfaceType())
+        if (auto *funcType = decl->getInterfaceType()->getAs<AnyFunctionType>())
+          candidates.emplace_back(funcType, decl);
     }
   } else if (auto *UDE = dyn_cast<UnresolvedDotExpr>(fnExpr)) {
     collectPossibleCalleesByQualifiedLookup(