[clang-tools-extra] [clang-doc] add async loading (PR #93276)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `clang-tools-extra` at step 4 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/863

Here is the relevant piece of the build log for the reference:
```
Step 4 (clean-build-dir) failure: Delete failed. (failure)
Step 6 (build-unified-tree) failure: build (failure)
...
[663/4660] Building IntrinsicsHexagon.h...
[664/4660] Building DiagnosticSerializationKinds.inc...
[665/4660] Building IntrinsicsDirectX.h...
[666/4660] Building DiagnosticGroups.inc...
[667/4660] Building arm_mve_builtins.inc...
[668/4660] Building IntrinsicsPowerPC.h...
[669/4660] Building arm_fp16.inc...
[670/4660] Building IntrinsicsR600.h...
[671/4660] Building AttrList.inc...
[672/4660] Linking CXX executable bin\FileCheck.exe
FAILED: bin/FileCheck.exe 
cmd.exe /C "cd . && "C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" -E vs_link_exe --intdir=utils\FileCheck\CMakeFiles\FileCheck.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100190~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100190~1.0\x64\mt.exe --manifests  -- C:\PROGRA~2\MICROS~1\2019\BUILDT~1\VC\Tools\MSVC\1429~1.301\bin\HostX64\x64\link.exe /nologo utils\FileCheck\CMakeFiles\FileCheck.dir\FileCheck.cpp.obj utils\FileCheck\CMakeFiles\FileCheck.dir\__\__\resources\windows_version_resource.rc.res  /out:bin\FileCheck.exe /implib:lib\FileCheck.lib /pdb:bin\FileCheck.pdb /version:0.0 /machine:x64 /STACK:10000000 /INCREMENTAL:NO /subsystem:console  lib\LLVMFileCheck.lib  lib\LLVMSupport.lib  psapi.lib  shell32.lib  ole32.lib  uuid.lib  advapi32.lib  ws2_32.lib  ntdll.lib  delayimp.lib  -delayload:shell32.dll  -delayload:ole32.dll  lib\LLVMDemangle.lib  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
LINK: command "C:\PROGRA~2\MICROS~1\2019\BUILDT~1\VC\Tools\MSVC\1429~1.301\bin\HostX64\x64\link.exe /nologo utils\FileCheck\CMakeFiles\FileCheck.dir\FileCheck.cpp.obj utils\FileCheck\CMakeFiles\FileCheck.dir\__\__\resources\windows_version_resource.rc.res /out:bin\FileCheck.exe /implib:lib\FileCheck.lib /pdb:bin\FileCheck.pdb /version:0.0 /machine:x64 /STACK:10000000 /INCREMENTAL:NO /subsystem:console lib\LLVMFileCheck.lib lib\LLVMSupport.lib psapi.lib shell32.lib ole32.lib uuid.lib advapi32.lib ws2_32.lib ntdll.lib delayimp.lib -delayload:shell32.dll -delayload:ole32.dll lib\LLVMDemangle.lib kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:bin\FileCheck.exe.manifest" failed (exit code 1104) with the following output:
LINK : fatal error LNK1104: cannot open file 'bin\FileCheck.exe'

[673/4660] Building AttrSubMatchRulesList.inc...
[674/4660] Building IntrinsicsS390.h...
[675/4660] Building IntrinsicsRISCV.h...
[676/4660] Building riscv_vector_builtins.inc...
[677/4660] Building RegularKeywordAttrInfo.inc...
[678/4660] Building IntrinsicsWebAssembly.h...
[679/4660] Building Builtins.inc...
[680/4660] Building BuiltinsBPF.inc...
[681/4660] Building IntrinsicsSPIRV.h...
[682/4660] Building arm_neon.inc...
[683/4660] Building IntrinsicsVE.h...
[684/4660] Building arm_mve_builtin_cg.inc...
[685/4660] Building IntrinsicsX86.h...
[686/4660] Building IntrinsicsXCore.h...
[687/4660] Building arm_sme_sema_rangechecks.inc...
[688/4660] Building arm_mve_builtin_sema.inc...
[689/4660] Building arm_sve_builtin_cg.inc...
[690/4660] Building arm_cde_builtin_aliases.inc...
[691/4660] Building arm_sme_builtins_za_state.inc...
[692/4660] Building arm_sme_streaming_attrs.inc...
[693/4660] Building arm_sve_typeflags.inc...
[694/4660] Building arm_sve_sema_rangechecks.inc...
[695/4660] Building arm_sve_streaming_attrs.inc...
[696/4660] Building riscv_sifive_vector_builtin_sema.inc...
[697/4660] Building arm_sme_builtins.inc...
[698/4660] Building arm_cde_builtins.inc...
[699/4660] Building arm_cde_builtin_cg.inc...
[700/4660] Building riscv_sifive_vector_builtins.inc...
[701/4660] Building riscv_sifive_vector_builtin_cg.inc...
[702/4660] Building AttrParserStringSwitches.inc...
[703/4660] Building AttrSubMatchRulesParserStringSwitches.inc...
[704/4660] Building AttrTemplateInstantiate.inc...
[705/4660] Building AttrParsedAttrList.inc...
[706/4660] Building AttrParsedAttrKinds.inc...

```

https://github.com/llvm/llvm-project/pull/93276