[clang-tools-extra] the message in a static_assert is not always a string literal

Fixes build failure introduce by 47ccfd7.

Author: cor3ntin

clang-tools-extra/clang-tidy/modernize/UnaryStaticAssertCheck.cpp

@@ -21,7 +21,8 @@ void UnaryStaticAssertCheck::registerMatchers(MatchFinder *Finder) {
 void UnaryStaticAssertCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *MatchedDecl =
       Result.Nodes.getNodeAs<StaticAssertDecl>("static_assert");
-  const StringLiteral *AssertMessage = MatchedDecl->getMessage();
+  const auto *AssertMessage =
+      dyn_cast_if_present<StringLiteral>(MatchedDecl->getMessage());
 
   SourceLocation Loc = MatchedDecl->getLocation();