Update debug info test cases with empty SplitDebugFilename field.

This could be 'null' or the empty string, DIDescriptor::getStringField
coalesces the two cases anyway so it's just a matter of legible/efficient
representation.

The change in behavior of the DICompileUnit::get* functions could be
subsumed by the full verification check - but ideally that should just be an
assertion if we could front-load the actual debug info metadata failure paths.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176907 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/PowerPC/dbg.ll b/test/CodeGen/PowerPC/dbg.ll
index 8d87cf7..125f517 100644
--- a/test/CodeGen/PowerPC/dbg.ll
+++ b/test/CodeGen/PowerPC/dbg.ll
@@ -16,7 +16,7 @@
 
 !llvm.dbg.cu = !{!0}
 
-!0 = metadata !{i32 720913, i32 0, i32 12, metadata !"dbg.c", metadata !"/src", metadata !"clang version 3.1", i1 true, i1 true, metadata !"", i32 0, metadata !1, metadata !1, metadata !3, metadata !1} ; [ DW_TAG_compile_unit ]
+!0 = metadata !{i32 720913, i32 0, i32 12, metadata !"dbg.c", metadata !"/src", metadata !"clang version 3.1", i1 true, i1 true, metadata !"", i32 0, metadata !1, metadata !1, metadata !3, metadata !1, metadata !""} ; [ DW_TAG_compile_unit ]
 !1 = metadata !{i32 0}
 !3 = metadata !{metadata !5}
 !5 = metadata !{i32 720942, i32 0, metadata !6, metadata !"main", metadata !"main", metadata !"", metadata !6, i32 1, metadata !7, i1 false, i1 true, i32 0, i32 0, null, i32 256, i1 true, i32 (i32, i8**)* @main, null, null, metadata !13} ; [ DW_TAG_subprogram ]