[clang] Fix -Wunused-lambda-capture in TokenAnnotator.cpp (NFC)

llvm-project/clang/lib/Format/TokenAnnotator.cpp:2707:43:
error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
    auto IsQualifiedPointerOrReference = [this](FormatToken *T) {
                                          ^~~~
1 error generated.

clang/lib/Format/TokenAnnotator.cpp

@@ -2707,6 +2707,7 @@ class AnnotatingParser {
     auto IsQualifiedPointerOrReference = [this](FormatToken *T) {
       // This is used to handle cases such as x = (foo *const)&y;
       assert(!T->isTypeName(IsCpp) && "Should have already been checked");
+      (void)IsCpp; // Avoid -Wunused-lambda-capture when assertion is disabled.
       // Strip trailing qualifiers such as const or volatile when checking
       // whether the parens could be a cast to a pointer/reference type.
       while (T) {