[cpplint] Fix bug with "if constexpr"
[cpplint] Fix bug with "if constexpr"

This snippet of code:

    if constexpr (n == 1) {
        return 2;
    } else if constexpr (n == 2) {
      int x = 2;
      int y = 2;
      return x + y;
    }

was triggering the warning:

  If/else bodies with multiple statements require braces [readability/braces]


And, in general, cpplint.py was assuming that `if (cond)` was
the only `if` construction to expect, forgetting about
`if constexpr(cond)`.

Change-Id: I4cdc8e7f134c1ebd14d00354a8baadf87c796763
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3644147
Reviewed-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Commit-Queue: Darius Mercadier <7faa01fd570b561b63eea98176a0e6fe517e0e00@chromium.org>
Reviewed-by: Jochen Eisinger <a43c420013ca055cc92ab03cc74d6f3bd5524c38@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools