Use proper check for FunctionProtoType

...as at least MSVC SAL_CALL-annotated functions have an AttributeType wrapped
around the FunctionProtoType.

Change-Id: Ic085e2e3649e6b2fc8ca380047133a8edbe20589

compilerplugins/clang/dynexcspec.cxx

@@ -47,7 +47,7 @@ public:
         if (ignoreLocation(decl)) {
             return true;
         }
-        auto proto = dyn_cast<FunctionProtoType>(decl->getType());
+        auto proto = decl->getType()->getAs<FunctionProtoType>();
         if (proto == nullptr || proto->getExceptionSpecType() != EST_Dynamic) {
             return true;
         }