Add build step for ICPC (with fixes) (#3465)

* ⬆️ Doctest 2.4.7

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* ⬇️ downgrade to Doctest 2.4.6

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* 👷 add CI step for ICPC

* 🔇 suppress warning #2196: routine is both "inline" and "noinline"

* Re-enable <filesystem> detection on ICPC

* Limit regression test for #3070 to Clang and GCC >=8.4

* Disable deprecation warnings on ICPC

* Disable regression test for #1647 on ICPC (C++20)

* Fix compilation failure of regression test for #3077 on ICPC

* Disable wstring unit test on ICPC

Fixes:
  error 913: invalid multibyte character sequence

* Add ICPC to README

Co-authored-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -112,3 +112,15 @@ jobs:
         run: cmake -S . -B build -DJSON_CI=On
       - name: build
         run: cmake --build build --target ci_cuda_example
+
+  ci_icpc:
+    runs-on: ubuntu-latest
+    container: ghcr.io/nlohmann/json-ci:v2.2.0
+    steps:
+      - uses: actions/checkout@v2
+      - name: cmake
+        run: cmake -S . -B build -DJSON_CI=On
+      - name: build
+        run: |
+          . /opt/intel/oneapi/setvars.sh
+          cmake --build build --target ci_icpc