[NFC] Explicitly use clang::Module in test file

#122887 uses `Module` to refer
to `clang::Module` in a test that has `using namespace llvm;`.

This causes lookup ambiguity with `llvm::Module` if the headers involved
expose that name (e.g., for downstream codebases).

Author: hubert-reinterpretcast

clang/unittests/AST/ExternalASTSourceTest.cpp

@@ -69,7 +69,7 @@ TEST(ExternalASTSourceTest, FailedLookupOccursOnce) {
 
     bool FindExternalVisibleDeclsByName(const DeclContext *,
                                         DeclarationName Name,
-                                        Module *NamedModule) override {
+                                        clang::Module *NamedModule) override {
       if (Name.getAsString() == "j")
         ++Calls;
       return false;