[llvm] [CI] Enable -Werror in pre-merge CI (PR #155627)

[Nikita Popov via llvm-commits]

nikic wrote:

```
FAILED: tools/mlir/lib/CAPI/IR/CMakeFiles/obj.MLIRCAPIIR.dir/BuiltinTypes.cpp.o
sccache /opt/llvm/bin/clang++ -DGTEST_HAS_RTTI=0 -DMLIR_CAPI_BUILDING_LIBRARY=1 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/mlir/lib/CAPI/IR -I/home/gha/actions-runner/_work/llvm-project/llvm-project/mlir/lib/CAPI/IR -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/mlir/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/mlir/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -Wundef -Werror=mismatched-tags -O3 -DNDEBUG -std=c++17 -fvisibility=hidden  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT tools/mlir/lib/CAPI/IR/CMakeFiles/obj.MLIRCAPIIR.dir/BuiltinTypes.cpp.o -MF tools/mlir/lib/CAPI/IR/CMakeFiles/obj.MLIRCAPIIR.dir/BuiltinTypes.cpp.o.d -o tools/mlir/lib/CAPI/IR/CMakeFiles/obj.MLIRCAPIIR.dir/BuiltinTypes.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/mlir/lib/CAPI/IR/BuiltinTypes.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/mlir/lib/CAPI/IR/BuiltinTypes.cpp:468:17: error: unused function 'mlirUnrankedTensorTypeGetElementType' [-Werror,-Wunused-function]
468 | static MlirType mlirUnrankedTensorTypeGetElementType(MlirType type) {
|                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

Looks like this warning was freshly introduced in https://github.com/llvm/llvm-project/commit/0d4b30e24f3f0543dadb54b6630d8752095b53cd.

https://github.com/llvm/llvm-project/pull/155627