AST: Rename GenericContext::isGeneric to hasGenericParamList

`isGeneric` is a misleading name because this method checks for the
existence of a `GenericParamList`, which is not implied by genericity.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -610,7 +610,7 @@ public:
       bool imported = false;
       if (TAD->hasClangNode())
         imported = addImport(TD);
-      assert((imported || !TAD->isGeneric()) &&
+      assert((imported || !TAD->hasGenericParamList()) &&
              "referencing non-imported generic typealias?");
     } else if (addImport(TD)) {
       return;
@@ -1109,7 +1109,7 @@ public:
             vd, [this](const NominalTypeDecl *decl) {
               return printer.isZeroSized(decl);
             });
-        if (nmtd->isGeneric()) {
+        if (nmtd->hasGenericParamList()) {
           auto genericSignature =
               nmtd->getGenericSignature().getCanonicalSignature();
           ClangSyntaxPrinter(nmtd->getASTContext(), os).printGenericSignature(genericSignature);