[torque] Improve error message in constexpr conditionals

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}

src/torque/implementation-visitor.cc

@@ -565,7 +565,8 @@ const Type* ImplementationVisitor::Visit(IfStatement* stmt) {
 
     if (!(expression_result.type() == TypeOracle::GetConstexprBoolType())) {
       std::stringstream stream;
-      stream << "expression should return type \"constexpr bool\" but doesn't";
+      stream << "expression should return type constexpr bool "
+             << "but returns type " << expression_result.type();
       ReportError(stream.str());
     }