We want Vk::Stringify to return the numeric value passed to it when it's
in release mode or when it fails to find the matching string. However,
we can't do that if we return a char* because the char* will be a
pointer to a point on the stack.

Bug: b/139528538
Change-Id: I1bab741e7e7aceea44c6179b2159f8a55497d3e0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38868Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Sean Risser <srisser@google.com>
Presubmit-Ready: Sean Risser <srisser@google.com>