This CL adds the incorrect return type of the conditional expression
in 'if constexpr' conditionals to the error message displayed
if the return type is different from 'constexpr bool'.

Change-Id: I58be4c140fa6fc475dc907a9ec872005fcb7d839
Reviewed-on: https://chromium-review.googlesource.com/1105768Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Théotime Grohens <theotime@google.com>
Cr-Commit-Position: refs/heads/master@{#53839}