Suppress C5041 constexpr warning for MSVC 2019

Disable the C5041 warning which makes the build fail. This is a
C++17-only change and WebRTC doesn't support C++17 yet, so the code is
technically correct, but fails to build on MSVC 2019 and
warning-as-error active.

Also fix another warning-as-error build error with MSVC 2019 due to
ignoring the result of a [[nodiscard]] function.

No-Presubmit: True
Bug: webrtc:11275,webrtc:11276
Change-Id: I891a894ee87252f96e84fd8d282576f46907256f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165781
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30244}
diff --git a/modules/audio_coding/acm2/acm_receiver_unittest.cc b/modules/audio_coding/acm2/acm_receiver_unittest.cc
index 175d11f..74a0c7a 100644
--- a/modules/audio_coding/acm2/acm_receiver_unittest.cc
+++ b/modules/audio_coding/acm2/acm_receiver_unittest.cc
@@ -289,7 +289,8 @@
   constexpr int payload_type = 34;
   const SdpAudioFormat codec = {"L16", 16000, 1};
   const AudioCodecInfo info = SetEncoder(payload_type, codec);
-  encoder_factory_->QueryAudioEncoder(codec).value();
+  auto const value = encoder_factory_->QueryAudioEncoder(codec);
+  ASSERT_TRUE(value.has_value());
   receiver_->SetCodecs({{payload_type, codec}});
   const int kNumPackets = 5;
   AudioFrame frame;