Fix DISubprogram line number to point to function definition line

[jiel-nv]

Fixes incorrect DW_AT_decl_line where the line number was first_stmt_line - 1 instead of the actual def line, causing off-by-one errors when comments exist between the def line and first statement.

• Add get_func_def_lineno() in firstlinefinder.py to return the actual 'def' line
• Update lowering.py to use get_func_def_lineno() directly without offset calculation
• Add test_DISubprogram_def_line_number covering single/multi-line decorators with/without comments

This change resolves nvbug#5723821.

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[greptile-apps]

Greptile Summary

This PR fixes an off-by-one error in debug information where DW_AT_decl_line was incorrectly pointing to first_stmt_line - 1 instead of the actual function definition line.

Changes:

• Added get_func_def_lineno() function in firstlinefinder.py that returns the actual def line number from the AST
• Modified FindDefFirstLine class to capture def_lineno (line 31, 51)
• Updated lowering.py to use get_func_def_lineno() directly without offset calculation (removed the -1 adjustment)
• Added comprehensive test test_DISubprogram_def_line_number covering single/multi-line decorators with/without comments

The fix ensures that debug information correctly points to the function definition line regardless of comments between the def line and the first statement.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is straightforward and well-tested. It fixes a specific bug in debug information generation by using the correct line number from the AST. The new function mirrors the existing get_func_body_first_lineno() pattern, comprehensive tests cover multiple scenarios (single/multi-line decorators, with/without comments), and the change is isolated to debug information generation without affecting runtime behavior
• No files require special attention

Important Files Changed

Filename	Overview
numba_cuda/numba/cuda/misc/firstlinefinder.py	Adds get_func_def_lineno() function to return the actual 'def' line number from AST, and stores def_lineno in FindDefFirstLine class
numba_cuda/numba/cuda/lowering.py	Switches from get_func_body_first_lineno() to get_func_def_lineno() and removes the -1 offset calculation
numba_cuda/numba/cuda/tests/cudapy/test_debuginfo.py	Adds comprehensive test coverage for DISubprogram line numbers with various decorator and comment configurations

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[jiel-nv]

/ok to test 6fe1b2f

[jiel-nv]

/ok to test 8286bc0

[gmarkall]

Apologies for the delay in reviewing. Whilst testing it, I discovered and investigated #721, which I think needs fixing at some point but could be done separately from this PR. I think this PR is otherwise correct and makes an improvement.