[llvm] [lit] Implement builtin umask (PR #94621)

[LLVM Continuous Integration via llvm-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 `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/1329

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lit :: shtest-umask.py' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 3
not env -u FILECHECK_OPTS "C:\Program Files\Python310\python.exe" Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\utils\lit\lit.py -j1 --order=lexical -a -v Inputs/shtest-umask | FileCheck -match-full-lines Z:\b\llvm-clang-x86_64-sie-win\build\utils\lit\tests\shtest-umask.py
# executed command: not env -u FILECHECK_OPTS 'C:\Program Files\Python310\python.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\utils\lit\lit.py' -j1 --order=lexical -a -v Inputs/shtest-umask
# executed command: FileCheck -match-full-lines 'Z:\b\llvm-clang-x86_64-sie-win\build\utils\lit\tests\shtest-umask.py'
# .---command stderr------------
# | Z:\b\llvm-clang-x86_64-sie-win\build\utils\lit\tests\shtest-umask.py:9:10: error: CHECK: expected string not found in input
# | # CHECK: # | Error: 'umask': invalid literal {{.*}}
# |          ^
# | <stdin>:9:10: note: scanning from here
# | umask bad
# |          ^
# | <stdin>:10:7: note: possible intended match here
# | # executed command: umask bad
# |       ^
# | Z:\b\llvm-clang-x86_64-sie-win\build\utils\lit\tests\shtest-umask.py:13:10: error: CHECK: expected string not found in input
# | # CHECK: # | 'umask' supports only one argument
# |          ^
# | <stdin>:29:10: note: scanning from here
# | umask 0 0
# |          ^
# | <stdin>:32:1: note: possible intended match here
# | # | 'umask' not supported on this system
# | ^
# | 
# | Input file: <stdin>
# | Check file: Z:\b\llvm-clang-x86_64-sie-win\build\utils\lit\tests\shtest-umask.py
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |             1: -- Testing: 3 tests, 1 workers -- 
# | check:5        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |             2: FAIL: shtest-umask :: umask-bad-arg.txt (1 of 3) 
# | label:7'0      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# | label:7'1      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |             3: ******************** TEST 'shtest-umask :: umask-bad-arg.txt' FAILED ******************** 
# |             4: Exit Code: 127 
# |             5:  
# |             6: Command Output (stdout): 
# |             7: -- 
# |             8: # RUN: at line 1 
# |             9: umask bad 
# | check:8        ^~~~~~~~~
# | check:9'0               X error: no match found
...

```

https://github.com/llvm/llvm-project/pull/94621