[llvm] [Support] Optimize parallel `TaskGroup` (PR #189196)

[via llvm-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>lib/Support/CMakeFiles/LLVMSupport.dir/Parallel.cpp.o</summary>

```
FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/Parallel.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/lib/Support -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/../third-party/siphash/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 -Werror=global-constructors -O3 -DNDEBUG -std=c++17 -UNDEBUG -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/Parallel.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/Parallel.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/Parallel.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Parallel.cpp
In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Parallel.cpp:9:
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/Support/Parallel.h:61:25: error: implicit instantiation of undefined template 'std::atomic<unsigned int>'
61 |   std::atomic<uint32_t> Count;
|                         ^
/usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/atomic_base.h:174:12: note: template is declared here
174 |     struct atomic;
|            ^
In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Parallel.cpp:9:
In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/Support/Parallel.h:20:
/usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/condition_variable:104:9: error: invalid argument type 'void' to unary expression
104 |         while (!__p())
|                ^~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/Support/Parallel.h:81:10: note: in instantiation of function template specialization 'std::condition_variable::wait<(lambda at /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/Support/Parallel.h:81:21)>' requested here
81 |     Cond.wait(lock, [&] { return Count.load(std::memory_order_acquire) == 0; });
|          ^
2 errors 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/189196