Replace SkSL string macros with inline constexpr char arrays.

C++17 introduced `inline constexpr` as a mechanism for declaring strings
in headers safely without macros. See https://abseil.io/tips/168

Change-Id: I68071fbadebf32753ca4a0586b96e32634083f6a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309047
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed