[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[via llvm-commits]

Prabhuk wrote:

We are seeing a test failure in our builders which I think is related to this patch:

https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-base-mac-x64/b8738251281152890817/overview

```
FAIL: LLVM :: DebugInfo/X86/fake-use.ll (28733 of 54710)
******************** TEST 'LLVM :: DebugInfo/X86/fake-use.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 6: /Volumes/Work/s/w/ir/x/w/llvm_build/bin/llc -O2 -filetype=obj -dwarf-linkage-names=Abstract < /Volumes/Work/s/w/ir/x/w/llvm-llvm-project/llvm/test/DebugInfo/X86/fake-use.ll | /Volumes/Work/s/w/ir/x/w/llvm_build/bin/llvm-dwarfdump --debug-info --debug-line -v - -o /Volumes/Work/s/w/ir/x/w/llvm_build/test/DebugInfo/X86/Output/fake-use.ll.tmp
+ /Volumes/Work/s/w/ir/x/w/llvm_build/bin/llc -O2 -filetype=obj -dwarf-linkage-names=Abstract
+ /Volumes/Work/s/w/ir/x/w/llvm_build/bin/llvm-dwarfdump --debug-info --debug-line -v - -o /Volumes/Work/s/w/ir/x/w/llvm_build/test/DebugInfo/X86/Output/fake-use.ll.tmp
RUN: at line 7: "/Volumes/Work/s/w/ir/cache/macos_sdk/XCode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9" /Volumes/Work/s/w/ir/x/w/llvm-llvm-project/llvm/test/DebugInfo/X86/../Inputs/check-fake-use.py /Volumes/Work/s/w/ir/x/w/llvm_build/test/DebugInfo/X86/Output/fake-use.ll.tmp
+ /Volumes/Work/s/w/ir/cache/macos_sdk/XCode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9 /Volumes/Work/s/w/ir/x/w/llvm-llvm-project/llvm/test/DebugInfo/X86/../Inputs/check-fake-use.py /Volumes/Work/s/w/ir/x/w/llvm_build/test/DebugInfo/X86/Output/fake-use.ll.tmp
Traceback (most recent call last):
  File "/Volumes/Work/s/w/ir/x/w/llvm-llvm-project/llvm/test/DebugInfo/X86/../Inputs/check-fake-use.py", line 57, in <module>
    raise RuntimeError(f"Invalid location range list for 'b'")
RuntimeError: Invalid location range list for 'b'
```

I am investigating further to confirm. 


https://github.com/llvm/llvm-project/pull/86149