[llvm] [openmp] Remaining work necessary for GCC 15 (PR #157385)

[Patrick Simmons via llvm-commits]

linuxrocks123 wrote:

> Your version of libstdc++ is 15.1, mine is 15.2. What makes the difference might be [gcc-mirror/gcc at 0dbc588](https://github.com/gcc-mirror/gcc/commit/0dbc588acaa27a3a56bc9173bd577e1293f10046), but I think you might have exceptions enabled because in all cases throwing of `bad_variant_access` is protected by `_GLIBCXX_THROW_OR_ABORT`, i.e. the compiler will not see any use of `bad_variant_access` unless exceptions are enabled. Please check.
> 
> I would rather ask users to update their version of libstdc++ than making debugging impossible. Or at least only do it as a workaround for 15.1.

This is the command.  Since `-fno-exceptions` is being passed, that's probably not the problem.

```
clang++ --target=x86_64-unknown-linux-gnu -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_NO_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/psimmons/work/llvm-project/flang-rt/include -I/home/psimmons/work/llvm-project/flang-rt/../flang/include -I/home/psimmons/work/llvm-project/llvm/build/runtimes/runtimes-bins/flang-rt  -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -g -std=gnu++17 -O2 -fno-lto -fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -o CMakeFiles/flang_rt.runtime.static.dir/io-api-minimal.cpp.o -c /home/psimmons/work/llvm-project/flang-rt/lib/runtime/io-api-minimal.cpp
```

https://github.com/llvm/llvm-project/pull/157385