Specify the C and C++ standards explicitly for this test. This avoids
failures for drivers that default to older standards.
Differential Revision: https://reviews.llvm.org/D109857
\x{XXXX} \u{XXXX} and \o{OOOO} are accepted in all languages mode
in characters and string literals.
This is a feature proposed for both C++ (P2290R1) and C (N2785). The
papers have been seen by both committees but are not yet adopted into
either standard. However, they do have support from both committees.