Fixes to support building in -std=c++20 mode.

* Structs with user-declared constructors are no longer considered
  aggregates, so remove the declarations when possible
* Types of both arguments to "==" must match to avoid "ambiguous
  function call" warning
* Various types of math involving enums are deprecated, so replace with
  constexprs where necessary
* ABSL_CONST_INIT must be used on definition as well as declaration
* volatile memory may no longer be read from and written to by the same
  operator, so replace e.g. "n++" with "n = n + 1"
* Replace an outdated check for no_unique_address support with
  __has_cpp_attribute
* std::result_of(f(x)) has been removed, replace with
  std::invoke_result(f, x)

Bug: chromium:1284275
Change-Id: I77b366ab1da7eb2c1e4c825b2714417c31ee5903
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261221
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Tomas Gunnarsson <tommi@google.com>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36786}

system_wrappers/include/metrics.h

@@ -363,7 +363,7 @@ namespace webrtc {
 namespace metrics {
 
 // Time that should have elapsed for stats that are gathered once per call.
-enum { kMinRunTimeInSeconds = 10 };
+constexpr int kMinRunTimeInSeconds = 10;
 
 class Histogram;