Fix -Wreturn-type in ShadowSamplerFunctionsTest::FunctionName Add UNREACHABLE() and a fallback return to ensure the non-void function always returns when switch does not cover all enum values. BUG: angleproject:438226513 Change-Id: I3904dd59b323b90930d9251b060ab7067ff412c7 Signed-off-by: Sungyong Choi <sywow.choi@samsung.com> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6845486 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>