[Github] Enable warnings as errors on flang sphinx build (llvm#72723)

Now that the number of warnings in the flang sphinx build has come down
significantly, we can turn on warnings as errors in the sphinx build,
which is the default configuration in CMake.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -164,8 +164,6 @@ jobs:
           TZ=UTC ninja -C polly-build docs-polly-html docs-polly-man
       - name: Build Flang docs
         if: steps.docs-changed-subprojects.outputs.flang_any_changed == 'true'
-        # TODO(boomanaiden154): Remove the SPHINX_WARNINGS_AS_ERRORS from the
-        # CMake invocation once the warnings in the flang docs build are fixed.
         run: |
-          cmake -B flang-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_SPHINX=ON -DSPHINX_WARNINGS_AS_ERRORS=OFF ./llvm
-          TZ=UTC ninja -C flang-build docs-flang-html docs-flang-man
+          cmake -B flang-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_SPHINX=ON ./llvm
+          TZ=UTC ninja -C flang-build docs-flang-html