[clang-tools-extra] [clang-doc] Add a Markdown parser (PR #155887)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

The build failed before running any tests. Click on a failure below to see the details.

<details>
<summary>tools/clang/tools/extra/clang-doc/CMakeFiles/obj.clangDoc.dir/MDParser.cpp.o</summary>

```
FAILED: tools/clang/tools/extra/clang-doc/CMakeFiles/obj.clangDoc.dir/MDParser.cpp.o
sccache /opt/llvm/bin/clang++ -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/tools/extra/clang-doc -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang-tools-extra/clang-doc -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/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 -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -Wno-nested-anon-types -O3 -DNDEBUG -std=c++17 -UNDEBUG -fno-exceptions -funwind-tables -fno-rtti -MD -MT tools/clang/tools/extra/clang-doc/CMakeFiles/obj.clangDoc.dir/MDParser.cpp.o -MF tools/clang/tools/extra/clang-doc/CMakeFiles/obj.clangDoc.dir/MDParser.cpp.o.d -o tools/clang/tools/extra/clang-doc/CMakeFiles/obj.clangDoc.dir/MDParser.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/clang-tools-extra/clang-doc/MDParser.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang-tools-extra/clang-doc/MDParser.cpp:229:15: error: enumeration value 'Document' not handled in switch [-Werror,-Wswitch]
229 |       switch (CurrentBlockType) {
|               ^~~~~~~~~~~~~~~~
1 error generated.
```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/155887