Properly propagate is_variadic.

This fixes a copy&paste error made when adapting to new clang API
which was promptly caught by the bots.
diff --git a/lldb/source/Symbol/ClangASTContext.cpp b/lldb/source/Symbol/ClangASTContext.cpp
index 5c7000a..290ad93 100644
--- a/lldb/source/Symbol/ClangASTContext.cpp
+++ b/lldb/source/Symbol/ClangASTContext.cpp
@@ -8639,7 +8639,7 @@
     return nullptr;
 
   const bool isInstance = (name[0] == '-');
-  const bool isVariadic = false;
+  const bool isVariadic = is_variadic;
   const bool isPropertyAccessor = false;
   const bool isSynthesizedAccessorStub = false;
   /// Force this to true because we don't have source locations.