AST fuzzer: reduce depth for unary wrapping

Reduces the extent to which programs with unparsable expressions are
generated by limiting the recursion depth allowed for wrapping unary
operators. Also updates some nested namespaces to use more modern C++.

Change-Id: I4637c20c9c72c6b315c04c9322d069f0e35859b3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/85580
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Alastair Donaldson <afdx@google.com>
Auto-Submit: Alastair Donaldson <afdx@google.com>

src/tint/fuzzers/tint_ast_fuzzer/mutation_finders/wrap_unary_operators.cc

@@ -28,7 +28,7 @@ namespace fuzzers {
 namespace ast_fuzzer {
 
 namespace {
-const size_t kMaxExpressionSize = 100;
+const size_t kMaxExpressionSize = 50;
 }  // namespace
 
 MutationList MutationFinderWrapUnaryOperators::FindMutations(