[Flang] Fix a test case that depends on stderr output of nullptr.

[DanielCChen]

This test case depends on the stderr output of a nullptr being "(null)". However, it is empty string on AIX.
This PR is to initialize the sourceFileName_ to be a specific string to avoid that.

[DavidSpickett]

The specific filename doesn't seem to be important to the test, so this LGTM.

[kkwli]

LG

[DanielCChen]

The WindowX6\4 CI failed with

1700850812986 ********************
�_bk;t=1700850812987 UNRESOLVED: Flang :: Lower/OpenMP/declare-target-implicit-func-and-subr-cap.f90 (849 of 2420)
�_bk;t=1700850812987 ******************** TEST 'Flang :: Lower/OpenMP/declare-target-implicit-func-and-subr-cap.f90' FAILED ********************
�_bk;t=1700850812987 Exception during script execution:
�_bk;t=1700850812987 Traceback (most recent call last):
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\worker.py", line 76, in _execute_test_handle_errors
�_bk;t=1700850812987     result = test.config.test_format.execute(test, lit_config)
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\formats\shtest.py", line 29, in execute
�_bk;t=1700850812987     return lit.TestRunner.executeShTest(
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\TestRunner.py", line 2243, in executeShTest
�_bk;t=1700850812987     parsed = parseIntegratedTestScript(test, require_script=not script)
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\TestRunner.py", line 2105, in parseIntegratedTestScript
�_bk;t=1700850812987     parsed = _parseKeywords(
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\TestRunner.py", line 2044, in _parseKeywords
�_bk;t=1700850812987     for line_number, command_type, ln in parseIntegratedTestScriptCommands(
�_bk;t=1700850812987   File "C:\ws\src\llvm\utils\lit\lit\TestRunner.py", line 1280, in parseIntegratedTestScriptCommands
�_bk;t=1700850812987     f = open(source_path, "rb")
�_bk;t=1700850812987 OSError: [Errno 22] Invalid argument: 'C:\\ws\\src\\flang\\test\\Lower\\OpenMP\\declare-target-implicit-func-and-subr-cap.f90'

I am not sure how it is related to this PR change. I don't have a window machine to reproduce it.
Is there any way that I can re-run the CI on the agent?
I do a rebase push to trigger another CI build.