Fix jumpy debug behavior issue

[yixi]

Summary

When debugging an executable program completed by Clang, if you set a break to a function, it is often no way to stop right at the line of the function, but jump to the end of the program. For example:

Reading symbols from ./a.out...done.
(gdb) b main
Breakpoint 1 at 0x4005ec
(gdb) r
Starting program: /tmp/a.out

Breakpoint 1, 0x00000000004005ec in main ()
(gdb) n
Single stepping until exit from function main,
which has no line number information.
0x0000ffffbf559ae0 in __libc_start_main () from /usr/lib64/libc.so.6

GDB version: GNU gdb (GDB) 8.2

This patch is trying to solve the issue.

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

   for (const auto &MBB : *MF)
     for (const auto &MI : MBB)
       if (!MI.isMetaInstruction() && !MI.getFlag(MachineInstr::FrameSetup) &&
-          MI.getDebugLoc())
+          MI.getDebugLoc() && (MI.getDebugLoc().getLine() != 0))
         return MI.getDebugLoc();
   return DebugLoc();
 }

llvm/test/DebugInfo/AArch64/prologue_end_line_0.ll

+; Check whether prologue_end was marked correctly. The IR in this test is
+; reduced form this C program:
+;
+; int var1;
+; int main() {
+;   var1 = 2;
+;   return 0;
+; }
+
+; RUN: llc -O0 %s -o - | FileCheck %s
+
+; CHECK: .loc  1 0 0
+; CHECK: .loc  1 3 8 prologue_end
+
+@var1 = common dso_local global i32 0, align 4, !dbg !0
+
+; Function Attrs: noinline nounwind optnone
+define dso_local i32 @main() #0 !dbg !11 {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  store i32 2, i32* @var1, align 4, !dbg !14
+  ret i32 0, !dbg !15
+}
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "var1", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/tmp")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!11 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 2, type: !12, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !4)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!6}
+!14 = !DILocation(line: 3, column: 8, scope: !11)
+!15 = !DILocation(line: 4, column: 3, scope: !11)

llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir

 # RUN: llc -start-before=machine-cp -O2 -filetype=asm -mtriple=x86_64-apple-macosx10.9.0 -o - %s | FileCheck %s
 
 # CHECK: Ltmp0:
-# CHECK: .loc 1 0 0 prologue_end
-# CHECK-NOT: .loc 1 0 0
-# CHECK: .loc 1 37 1
+# CHECK: .loc 1 0 0
+# CHECK: .loc 1 37 1 prologue_end
 
 --- |
   ; ModuleID = '<stdin>'

llvm/test/DebugInfo/X86/dbg-prolog-end.ll

 declare void @llvm.dbg.declare(metadata, metadata, metadata) nounwind readnone
 
 ;CHECK-LABEL: main:
-;CHECK: .loc 1 0 0 prologue_end
+;CHECK: .loc 1 8 2 prologue_end
 
 define i32 @main() nounwind ssp !dbg !6 {
 entry: