[DebugInfo] Fix line 0 records incorrectly having is_stmt set

Fixes issue #33870

Line 0 debug records should not have the is_stmt flag set in DWARF.
This change ensures is_stmt is only set for non-zero line numbers.

Changes made in DwarfDebug.cpp:
- beginInstruction: Only set is_stmt for prologue_end if line != 0
- beginInstruction: Only set is_stmt for key instructions if line != 0
- emitInitialLocDirective: Set is_stmt=0 if scopeLine == 0

Test results: All 576 X86 DebugInfo tests pass.

Author: RoshanYSingh23