[mlir][debug] Allow global with local scope. (llvm#98358)

Currently, only those global variables in debug info are allowed which
have a compile unit or module scope. But global variable with function
scope are used in debug info to represent certain type of variables. One
example will be static local variables in C. Here the variable is
represented as global variable with parent function as its scope. See
the code and debug metadata as generated by clang below. You can see
that scope of `DIGlobalVariable` is `DISubprogram`.

```
int test() {
    static int a = 0;
    return a++;
}

!0 = !DIGlobalVariableExpression(var: !1...)
!1 = !DIGlobalVariable(name: "a", scope: !2 ...)
!2 = !DISubprogram(name: "test" ...)
!7 = !DICompileUnit(... globals: !8...)
!8 = !{!0}
```

Similar issue exist in fortran where global variable with function scope
are used to represent local variable with save attribute.

This PR will allows such variables during module translation.

Author: abidh

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -1073,9 +1073,15 @@ LogicalResult ModuleTranslation::convertGlobals() {
       // variable -> module -> compile unit
       // If a variable scope points to the module then we use the scope of the
       // module to get the compile unit.
+      // Global variables are also used for things like static local variables
+      // in C and local variables with the save attribute in Fortran. The scope
+      // of the variable is the parent function. We use the compile unit of the
+      // parent function in this case.
       llvm::DIScope *scope = diGlobalVar->getScope();
-      if (llvm::DIModule *mod = dyn_cast_if_present<llvm::DIModule>(scope))
+      if (auto *mod = dyn_cast_if_present<llvm::DIModule>(scope))
         scope = mod->getScope();
+      else if (auto *sp = dyn_cast_if_present<llvm::DISubprogram>(scope))
+        scope = sp->getUnit();
 
       // Get the compile unit (scope) of the the global variable.
       if (llvm::DICompileUnit *compileUnit =

mlir/test/Target/LLVMIR/llvmir-debug.mlir

@@ -347,6 +347,25 @@ llvm.mlir.global external @module_global() {dbg_expr = #llvm.di_global_variable_
 
 // -----
 
+// CHECK: @func_global = external global i64, !dbg {{.*}}
+// CHECK-DAG: ![[CU:.*]] = distinct !DICompileUnit({{.*}}globals: ![[GVALS:.*]])
+// CHECK-DAG: ![[SP:.*]] = distinct !DISubprogram(name: "fn_with_gl"{{.*}}unit: ![[CU]])
+// CHECK-DAG: ![[GVAR:.*]] = distinct !DIGlobalVariable(name: "func_global"{{.*}}, scope: ![[SP]]{{.*}})
+// CHECK-DAG: ![[GEXPR:.*]] = !DIGlobalVariableExpression(var: ![[GVAR]], expr: !DIExpression())
+// CHECK-DAG: ![[GVALS]] = !{![[GEXPR]]}
+
+#file = #llvm.di_file<"test.f90" in "existence">
+#cu = #llvm.di_compile_unit<id = distinct[0]<>, sourceLanguage = DW_LANG_Fortran95, file = #file, producer = "MLIR", isOptimized = true, emissionKind = Full>
+#ty1 = #llvm.di_basic_type<tag = DW_TAG_base_type, name = "integer", sizeInBits = 64, encoding = DW_ATE_signed>
+#sp = #llvm.di_subprogram<compileUnit = #cu, scope = #file, name = "fn_with_gl", file = #file, subprogramFlags = "Definition|Optimized">
+llvm.mlir.global @func_global() {dbg_expr = #llvm.di_global_variable_expression<var = <scope = #sp, name = "func_global", linkageName = "func_global", file = #file, line = 121, type = #ty1, isLocalToUnit = true, isDefined = true>, expr = <>>} : i64
+
+llvm.func @fn_with_gl() {
+  llvm.return
+} loc(fused<#sp>["foo1.mlir":0:0])
+
+// -----
+
 // Nameless and scopeless global constant.
 
 // CHECK-LABEL: @.str.1 = external constant [10 x i8]