[clang-tools-extra] [clang-doc] Add a Mustache Markdown generator (PR #177221)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-multistage` running on `ppc64le-clang-multistage-test` while building `clang-tools-extra` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/14108

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'SanitizerCommon-asan-powerpc64le-Linux :: Linux/getpwnam_r_invalid_user.cpp' FAILED ********************
Exit Code: -6

Command Output (stdout):
--
# RUN: at line 2
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/./bin/clang  --driver-mode=g++ -gline-tables-only -fsanitize=address  -m64 -fno-function-sections -funwind-tables  -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/compiler-rt/test -ldl -O0 -g /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/compiler-rt/test/sanitizer_common/TestCases/Linux/getpwnam_r_invalid_user.cpp -o /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-powerpc64le-Linux/Linux/Output/getpwnam_r_invalid_user.cpp.tmp &&  /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-powerpc64le-Linux/Linux/Output/getpwnam_r_invalid_user.cpp.tmp
# executed command: /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/./bin/clang --driver-mode=g++ -gline-tables-only -fsanitize=address -m64 -fno-function-sections -funwind-tables -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/compiler-rt/test -ldl -O0 -g /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/compiler-rt/test/sanitizer_common/TestCases/Linux/getpwnam_r_invalid_user.cpp -o /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-powerpc64le-Linux/Linux/Output/getpwnam_r_invalid_user.cpp.tmp
# executed command: /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-powerpc64le-Linux/Linux/Output/getpwnam_r_invalid_user.cpp.tmp
# .---command stderr------------
# | Result: 110
# | getpwnam_r_invalid_user.cpp.tmp: /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/compiler-rt/test/sanitizer_common/TestCases/Linux/getpwnam_r_invalid_user.cpp:19: int main(): Assertion `res == 0 || res == ENOENT' failed.
# `-----------------------------
# error: command failed with exit status: -6

--

********************

Step 10 (build stage 2) failure: 'ninja' (failure) (timed out)
...
[6387/6712] Linking CXX executable bin/llvm-lipo
[6388/6712] Linking CXX executable bin/llvm-dwarfutil
[6389/6712] Linking CXX executable bin/clang-sycl-linker
[6390/6712] Linking CXX executable bin/llvm-isel-fuzzer
[6391/6712] Linking CXX executable bin/llvm-reduce
[6392/6712] Linking CXX executable bin/llvm-c-test
[6393/6712] Linking CXX executable bin/llvm-opt-fuzzer
[6394/6712] Linking CXX executable bin/llvm-split
[6395/6712] Linking CXX executable bin/opt
[6396/6712] Linking CXX executable bin/llvm-ir2vec
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3011.135774

```

</details>

https://github.com/llvm/llvm-project/pull/177221