Reserve a vector where we know the size. (#22077)

* Reserve a vector where we know the size.

* Made a temp. to save the cost of calling NTD->getAllProtocols() twice.

Author: shahzadlone

tools/swift-ide-test/ModuleAPIDiff.cpp

@@ -769,8 +769,10 @@ class SMAModelGenerator : public DeclVisitor<SMAModelGenerator> {
   }
 
   std::vector<sma::TypeName> collectProtocolConformances(NominalTypeDecl *NTD) {
+    const auto AllProtocols = NTD->getAllProtocols();
     std::vector<sma::TypeName> Result;
-    for (const auto *PD : NTD->getAllProtocols()) {
+    Result.reserve(AllProtocols.size());
+    for (const auto *PD : AllProtocols) {
       Result.emplace_back(convertToTypeName(PD->getDeclaredType()));
     }
     return Result;