[flang][debug] Use DICompileUnit as scope.

This PR is required for llvm#96316. For assumed size arrays, we don't have
the size of the array in the last dimension. This is represented in
metadata by having a DISubrange with no entry for count or upper bound.

LLVM verifier allows a DISubrange with no count or upper bound only for
Fortran. This language is set when DICompileUnit is processed. If a type
is processed before DICompileUnit then Verifier will not allow
DISubrange with not count or upper bound entry. It will fail with error
"Subrange must contain count or upperBound".

Above scenario easily happens if a subroutine at global scope takes
assumed size parameter. The metadata on DISubprogram is such that type
is processed before unit so we hit Verifier::visitDISubrange before
language has been set to Fortran.

I have worked around this issue by setting the scope of such functions
to be DICompileUnit instead of DIFile. As scope metadata in
DISubprogram is processed before type so this problem does not
happen.

Some other observations about this change:
1. It does not make any difference to the generated DWARF.
2. It bring subroutine inline with module as modules already set their
scope to DiCompileUnit.
3. I also looked at the classic flang compiler and it also uses
DICompileUnit as scope for such subroutines probably for the same
reasons.

Author: abidh

flang/lib/Optimizer/Transforms/AddDebugInfo.cpp

@@ -266,7 +266,7 @@ void AddDebugInfoPass::runOnOperation() {
 
     // Only definitions need a distinct identifier and a compilation unit.
     mlir::DistinctAttr id;
-    mlir::LLVM::DIScopeAttr Scope = fileAttr;
+    mlir::LLVM::DIScopeAttr Scope = cuAttr;
     mlir::LLVM::DICompileUnitAttr compilationUnit;
     mlir::LLVM::DISubprogramFlags subprogramFlags =
         mlir::LLVM::DISubprogramFlags{};

flang/test/Transforms/debug-line-table-inc-file.fir

@@ -31,7 +31,7 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<>} {
 // CHECK: #[[LOC_INC_FILE:.*]] = loc("{{.*}}inc.f90":1:1)
 // CHECK: #[[LOC_FILE:.*]] = loc("{{.*}}simple.f90":3:1)
 // CHECK: #[[DI_CU:.*]] = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #[[DI_FILE]], producer = "{{.*}}flang{{.*}}", isOptimized = false, emissionKind = LineTablesOnly>
-// CHECK: #[[DI_SP_INC:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_FILE]], name = "sinc", linkageName = "_QPsinc", file = #[[DI_INC_FILE]], {{.*}}>
-// CHECK: #[[DI_SP:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_FILE]], name = "_QQmain", linkageName = "_QQmain", file = #[[DI_FILE]], {{.*}}>
+// CHECK: #[[DI_SP_INC:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_CU]], name = "sinc", linkageName = "_QPsinc", file = #[[DI_INC_FILE]], {{.*}}>
+// CHECK: #[[DI_SP:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_CU]], name = "_QQmain", linkageName = "_QQmain", file = #[[DI_FILE]], {{.*}}>
 // CHECK: #[[FUSED_LOC_INC_FILE]] = loc(fused<#[[DI_SP_INC]]>[#[[LOC_INC_FILE]]])
 // CHECK: #[[FUSED_LOC_FILE]] = loc(fused<#[[DI_SP]]>[#[[LOC_FILE]]])