feat(Core): Enable C++20 support (#10440)

* feat(Core): Enable C++20 support

* Update Duration.h

* Revert "Update Duration.h"

This reverts commit 177093e992c5d47d8c3b978c84857f5ecba12889.

* maybe fix GCC

* cherry-pick https://gcc.gnu.org/pipermail/gcc-cvs/2020-June/299715.html

* Update Duration.h

* Update Duration.h

* Update Duration.h

* Update Duration.h

* Update Duration.h

* Update Duration.h

* Update Duration.h

* Revert "Update Duration.h"

This reverts commit dc4e2ce281dd1a33cfac5be56488a3b96131bfd5.

* Update Duration.h

* Update Duration.h

* Update Duration.h

* cleanup

* more cleanup

* maybe fix build

* restore advstd::type_identity because GCC8 is garbage

* Update advstd.h

* fix gcc8 💤

* Update CMakeLists.txt

* Update CMakeLists.txt

* Update src/common/Utilities/advstd.h

Co-authored-by: Francesco Borzì <borzifrancesco@gmail.com>

.github/workflows/cpp20.yml

@@ -1,46 +0,0 @@
-name: C++20
-on:
-  push:
-    branches:
-      - 'master'
-  pull_request:
-    types: ['labeled', 'labeled', 'opened', 'synchronize', 'reopened']
-
-concurrency:
-  group: ${{ github.head_ref }} || concat(${{ github.ref }}, ${{ github.workflow }})
-  cancel-in-progress: true
-
-jobs:
-  build:
-    runs-on: ubuntu-20.04
-    name: C++20
-    env:
-      COMPILER: clang
-      ENABLE_CPP_20: 1
-    if: github.repository == 'azerothcore/azerothcore-wotlk' && (github.ref == 'refs/heads/master' || contains(github.event.pull_request.labels.*.name, 'file-cpp') || github.event.label.name == 'file-cpp' || contains(github.event.pull_request.labels.*.name, 'run-build') || github.event.label.name == 'run-build')
-    steps:
-      - uses: actions/checkout@v2
-      - name: Cache
-        uses: actions/cache@v2
-        with:
-          path: var/ccache
-          key: ccache:C++20:${{ github.ref }}:${{ github.sha }}
-          restore-keys: |
-            ccache:C++20:${{ github.ref }}
-            ccache:C++20
-      - name: Configure OS
-        run: source ./acore.sh install-deps
-        env:
-          CONTINUOUS_INTEGRATION: true
-      - name: Create conf/config.sh
-        run: source ./apps/ci/ci-conf.sh
-      - name: Import db
-        run: source ./apps/ci/ci-import-db.sh
-      - name: Build
-        run: source ./apps/ci/ci-compile.sh
-      - name: Dry run
-        run: source ./apps/ci/ci-worldserver-dry-run.sh
-      - name: Check startup errors
-        run: source ./apps/ci/ci-error-check.sh
-      - name: Run unit tests
-        run: source ./apps/ci/ci-run-unit-tests.sh