Tests: Protect unicode literals from clang-format Cpp03 formatting

Since CMake is written in C++98 any clang-format configuration must
set `Standard` to `Cpp03` so that `A<A<int> >` is not rewritten as
`A<A<int>>`.  However, this will cause `U"foo"` to be rewritten as
`U "foo"`.  Add markup to turn clang-format off in the one place
that the latter case occurs so that we do not need a separate
`.clang-format` config file for it.

Inspired-by: Daniel Pfeifer <daniel@pfeifer-mail.de>

Tests/CompileFeatures/cxx_unicode_literals.cpp

@@ -1,3 +1,5 @@
 
+/* clang-format off */
 const char16_t lit_16[] = u"\u00DA";
 const char32_t lit_32[] = U"\u00DA";
+/* clang-format on */