[clang-tools-extra] [clang-doc] Add helpers for Template config (PR #138062)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-rhel` running on `ppc64le-clang-rhel-test` while building `clang-tools-extra` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/145/builds/7205

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
...
0.197 [1/3/2] Building CXX object compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_dynamic_version_script_dummy.powerpc64le.dir/dummy.cpp.o
0.428 [0/3/3] Linking CXX shared library /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib/clang/21/lib/powerpc64le-unknown-linux-gnu/libclang_rt.ubsan_standalone.so
0.452 [0/2/4] Linking CXX shared library /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib/clang/21/lib/powerpc64le-unknown-linux-gnu/libclang_rt.asan.so
1.148 [0/1/5] Generating /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/compile_commands.json
25.587 [4/2/1212] cd /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/runtimes/runtimes-bins && /home/buildbots/llvm-external-buildbots/cmake-3.31.2/bin/cmake --build /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/runtimes/runtimes-bins/ --target runtimes-test-depends --config Release
ninja: no work to do.
25.655 [3/2/1213] No install step for 'runtimes'
25.681 [2/2/1215] Completed 'runtimes'
33.403 [2/1/1216] Building CXX object tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/HTMLMustacheGeneratorTest.cpp.o
33.462 [1/1/1217] Linking CXX executable tools/clang/tools/extra/unittests/clang-doc/ClangDocTests
FAILED: tools/clang/tools/extra/unittests/clang-doc/ClangDocTests 
: && /home/buildbots/llvm-external-buildbots/clang.19.1.7/bin/clang++ --gcc-toolchain=/gcc-toolchain/usr -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 -fno-common -Woverloaded-virtual -Wno-nested-anon-types -O3 -DNDEBUG -Wl,--color-diagnostics     -Wl,--gc-sections  -Xlinker --dependency-file=tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/link.d tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/BitcodeTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/ClangDocTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/GeneratorTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/HTMLGeneratorTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/HTMLMustacheGeneratorTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/MDGeneratorTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/MergeTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/SerializeTest.cpp.o tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/YAMLGeneratorTest.cpp.o -o tools/clang/tools/extra/unittests/clang-doc/ClangDocTests  -Wl,-rpath,/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib  lib/libllvm_gtest_main.so.21.0git  -lpthread  lib/libclangDoc.so.21.0git  lib/libLLVMTestingSupport.so.21.0git  lib/libclangTooling.so.21.0git  lib/libclangFormat.so.21.0git  lib/libclangFrontend.so.21.0git  lib/libclangSerialization.so.21.0git  lib/libclangToolingCore.so.21.0git  lib/libclangRewrite.so.21.0git  lib/libclangASTMatchers.so.21.0git  lib/libclangAST.so.21.0git  lib/libclangBasic.so.21.0git  lib/libLLVMFrontendOpenMP.so.21.0git  lib/libLLVMBitstreamReader.so.21.0git  lib/libllvm_gtest.so.21.0git  lib/libLLVMSupport.so.21.0git  -Wl,-rpath-link,/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib && :
ld.lld: error: undefined symbol: getMustacheHtmlFiles(llvm::StringRef, clang::doc::ClangDocContext&)
>>> referenced by HTMLMustacheGeneratorTest.cpp
>>>               tools/clang/tools/extra/unittests/clang-doc/CMakeFiles/ClangDocTests.dir/HTMLMustacheGeneratorTest.cpp.o:(HTMLMustacheGeneratorTest_generateDocs_Test::TestBody())
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/138062