Add DAWN_MULTILINE for minifying WGSL strings

And use it instead of raw strings in most places.

While doing that, also fixed the indentation of a bunch of strings (and
tried to make them look nice in the surrounding context) and removed a
bit of manually-added whitespace from generated strings, since generated
shaders are no longer nicely formatted at all.

Both clang-format and cpplint have minor issues with this approach
because they can't tell that the string is not C++ code.
- clang-format's WhitespaceSensitiveMacros option is buggy, so we
  need to pepper blank comments in a few places to prevent it from
  reformatting things it's not supposed to touch.
- cpplint has no option to disable for multiple lines, so one rule that
  triggers often on WGSL is easiest to just disable. Maybe this will
  not matter in the future, if we fully replace cpplint with clang-tidy.

In a local Skia build (with clang 17.0.0 from the system Xcode because
Skia isn't using a hermetic build on my Mac), this saves 22.3k:
  -159255840 out/DawnVulkan/libdawn_combined.a
  +159233520 out/DawnVulkan/libdawn_combined.a

Fixed: 475274568
Change-Id: Ibd70eefc7e314f8689bf06803e681a1ec10302d0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/284555
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>