GH Actions: install include-what-you-use, integrate it with CMake

See https://github.com/include-what-you-use/include-what-you-use/blob/93d8793081e5aa457114ef99d1e887390b370e4a/README.md#using-with-cmake

.github/workflows/build.yml

@@ -18,14 +18,28 @@ jobs:
           - '20'
     steps:
       - uses: actions/checkout@v4
-      - name: restore
+      - name: install GoogleTest
         run: |
           sudo apt-get update
           sudo apt-get install -y libgtest-dev
+      - name: install include-what-you-use (iwyu)
+        # NB: https://packages.ubuntu.com/jammy/iwyu apparently doesn't declare the `libclang-common-XXX-dev` package it
+        # needs as a dependency (without it, `include-what-you-use` fails with "fatal error: 'stddef.h' file not found"
+        # or similar), although this problem has been reported in 7 out of 7 bug reports at
+        # https://bugs.launchpad.net/ubuntu/+source/iwyu, the oldest one is from 2014.
+        #
+        # We intentionally specify an exact version of `iwyu` here, so that when a new version becomes available and we
+        # want to update to it, we'll have to change this hardcoded version manually and update the
+        # `libclang-common-XXX-dev` version accordingly (see
+        # https://github.com/include-what-you-use/include-what-you-use/blob/master/README.md#clang-compatibility).
+        run: sudo apt-get install -y iwyu=8.17-1 libclang-common-13-dev
       - name: build
         env:
           CPP_STANDARD: ${{ matrix.cpp-standard }}
-        run: .build/build -DCMAKE_CXX_STANDARD="$CPP_STANDARD" -DCMAKE_CXX_STANDARD_REQUIRED=ON -DCMAKE_CXX_EXTENSIONS=OFF
+        run: |
+          .build/build \
+            -DCMAKE_CXX_STANDARD="$CPP_STANDARD" -DCMAKE_CXX_STANDARD_REQUIRED=ON -DCMAKE_CXX_EXTENSIONS=OFF \
+            -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE='include-what-you-use;-Xiwyu;--verbose=3'
       - name: unittest
         run: .build/run-unittest