[mlir][llvm] Make DISubprogram name optional

[Dinistro]

Summary

This commit make the name parameter of the DISubprogramAttr optional.
LLVM will for example omit these subprogram names in initialization
functions for globals.

[gysit]

LGTM!

mlir/include/mlir/Dialect/LLVMIR/LLVMAttrDefs.td

 // DISubprogramAttr
 //===----------------------------------------------------------------------===//
 
 def LLVM_DISubprogramAttr : LLVM_Attr<"DISubprogram", "di_subprogram",
                                       /*traits=*/[], "DIScopeAttr"> {
   let parameters = (ins
     OptionalParameter<"DICompileUnitAttr">:$compileUnit,
     "DIScopeAttr":$scope,
-    "StringAttr":$name,
+    OptionalParameter<"StringAttr">:$name,
     OptionalParameter<"StringAttr">:$linkageName,
     "DIFileAttr":$file,
     OptionalParameter<"unsigned">:$line,
     OptionalParameter<"unsigned">:$scopeLine,
     "DISubprogramFlags":$subprogramFlags,
     OptionalParameter<"DISubroutineTypeAttr">:$type
   );
   let builders = [

mlir/test/Target/LLVMIR/Import/debug-info.ll

 !llvm.dbg.cu = !{!1}
 !llvm.module.flags = !{!0}
 !0 = !{i32 2, !"Debug Info Version", i32 3}
 !1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2)
 !2 = !DIFile(filename: "debug-info.ll", directory: "/")
 !7 = !DILocalVariable(scope: !8)
 !8 = distinct !DISubprogram(name: "noname_variable", scope: !2, file: !2, unit: !1);
 !9 = !DILocation(line: 1, column: 2, scope: !8)
+
+; // -----
+
+; CHECK: #[[SUBPROGRAM:.*]] = #llvm.di_subprogram<compileUnit = #{{.*}}, scope = #{{.*}}, file = #{{.*}}, subprogramFlags = Definition>
+; CHECK: #[[FUNC_LOC:.*]] = loc(fused<#[[SUBPROGRAM]]>[{{.*}}])
+define void @noname_subprogram(ptr %arg) !dbg !8 {
+  ret void
+}
+
+!llvm.dbg.cu = !{!1}
+!llvm.module.flags = !{!0}
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2)
+!2 = !DIFile(filename: "debug-info.ll", directory: "/")
+!8 = distinct !DISubprogram(scope: !2, file: !2, spFlags: DISPFlagDefinition, unit: !1);