[clang] [llvm] Reland: [llvm][clang] Allocate a new stack instead of spawning a new thread to get more stack space (PR #136046)

[Alan Phipps via cfe-commits]

evodius96 wrote:

Hello, I'm seeing ProgramStackTest fail when running on our downstream toolchain on Linux:
```
2 |   | FAIL: llvm_regressions :: LLVM-Unit/Support/SupportTests/ProgramStackTest/runOnNewStack
3 |   | --------------------------------------------------------------------------------
4 |   | Script:
5 |   | --
6 |   | /scratch/aphipps/triage_repo/tools/llvm_cgt/build/RelWithAsserts/llvm/unittests/Support/./SupportTests --gtest_filter=ProgramStackTest.runOnNewStack
7 |   | --
8 |   | /scratch/aphipps/triage_repo/tools/llvm_cgt/llvm-project/llvm/unittests/Support/ProgramStackTest.cpp:32
9 |   | Expected: (StackDistance) > (llvm::sys::Process::getPageSizeEstimate()), actual: 16 vs 4096
```
Not sure where to begin with debugging this.  Your commit says it should only apply to Apple?

https://github.com/llvm/llvm-project/pull/136046