[DwarfDebug] Avoid generating extra DW_TAG_subprogram entries (#154636)

The test llvm/test/DebugInfo/X86/pr12831.ll was added in 4d358b55fa 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. So,
another DIE is created for the same subprogram. In this PR, a check is
added to fix that.