[DwarfDebug] Restore broken test llvm/test/DebugInfo/X86/pr12831.ll

This test was added in 4d358b5 to fix the issue with emission of
empty DW_TAG_subprogram tags (https://bugs.llvm.org/show_bug.cgi?id=12831).

However, the test output is not checked properly, and it contains:
```
0x00000206:   DW_TAG_subprogram

0x00000207:   DW_TAG_reference_type
                DW_AT_type      (0x00000169 "class ")
```

The reason is that the DIE for the definition DISubprogram "writeExpr"
is created during the call to `getOrCreateSubprogramDIE(declaration of writeExpr)`.
Therefore, when `getOrCreateSubprogramDIE(definition of writeExpr)` is
first called, we get a recursive chain of calls:
```
getOrCreateSubprogramDIE(definition of writeExpr)
...
getOrCreateSubprogramDIE(declaration of writeExpr)
...
getOrCreateSubprogramDIE(definition of writeExpr)
```

The outer call doesn't expect that the DIE for the definition of
writeExpr will be created during the creation of declaration DIE.
In this PR, a check is added to fix that.

I'm not sure whether the problem described in
https://bugs.llvm.org/show_bug.cgi?id=12831 is still relevant (I wasn't
able to reproduce the problem with C++ reproducer from there).
But if this test stays in repo, it should be fixed.

Author: dzhidzhoev

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -1351,6 +1351,10 @@ DIE *DwarfUnit::getOrCreateSubprogramDIE(const DISubprogram *SP, bool Minimal) {
       ContextDIE = &getUnitDie();
       // Build the decl now to ensure it precedes the definition.
       getOrCreateSubprogramDIE(SPDecl);
+      // Check whether the DIE for SP has already been created after the call
+      // above.
+      if (DIE *SPDie = getDIE(SP))
+        return SPDie;
     }
   }
 

llvm/test/DebugInfo/X86/pr12831.ll

@@ -1,4 +1,39 @@
-; RUN: llc %s -mtriple=x86_64-unknown-linux-gnu -o /dev/null
+; RUN: llc %s -mtriple=x86_64-unknown-linux-gnu -filetype=obj -o - | llvm-dwarfdump - | FileCheck %s
+
+; Check that there are no empty DW_TAG_subprogram tags.
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT: DW_AT
+; CHECK-NOT: DW_TAG_subprogram
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"