Add a clang-tidy file

This gives us the ninja clang-tidy command. clang-tidy replaces
scan-build and is more featureful (and picky).

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1182>

.clang-tidy

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+# optin.core.EnumCastOutOfRange: disabled because we use a lot of "wrong" enum values for testing
+#                                and internally and don't want those values leak into the public API
+Checks: >
+  -clang-analyzer-optin.core.EnumCastOutOfRange
+WarningsAsErrors: true

.clang-tidy-ignore

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+include/*

test/.clang-tidy

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+# optin.core.FloatLoopCounter: doubles in for loops - it's fine in the tests
+# optin.performance.Padding: not a performance concern in the tests
+InheritParentConfig: true
+Checks: >
+  -clang-analyzer-security.FloatLoopCounter,
+  -clang-analyzer-optin.performance.Padding,