[clang-repl] Skip CodeGen for top-level decls when diagnostics report errors (#169989)

We can see the following while running clang-repl in C mode 
```
anutosh491@vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl --Xcc=-x --Xcc=c --Xcc=-std=c23
clang-repl> printf("hi\n");
In file included from <<< inputs >>>:1:
input_line_1:1:1: error: call to undeclared library function 'printf' with type 'int (const char *, ...)'; ISO C99 and
      later do not support implicit function declarations [-Wimplicit-function-declaration]
    1 | printf("hi\n");
      | ^
input_line_1:1:1: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
error: Parsing failed.
clang-repl> #include <stdio.h>
hi
```

In debug mode while dumping the generated Module, i see this 
```
clang-repl> printf("hi\n");
In file included from <<< inputs >>>:1:
input_line_1:1:1: error: call to undeclared library function 'printf' with type 'int (const char *, ...)'; ISO C99 and
      later do not support implicit function declarations [-Wimplicit-function-declaration]
    1 | printf("hi\n");
      | ^
input_line_1:1:1: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
error: Parsing failed.
clang-repl> #include <stdio.h>

=== compile-ptu 1 ===
[TU=0x55556cfbf830, M=0x55556cfc13a0 (incr_module_1)]
[LLVM IR]
; ModuleID = 'incr_module_1'
source_filename = "incr_module_1"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@.str = private unnamed_addr constant [4 x i8] c"hi\0A\00", align 1
@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @_GLOBAL__sub_I_incr_module_1, ptr null }]

define internal void @__stmts__0() #0 {
entry:
  %call = call i32 (ptr, ...) @printf(ptr noundef @.str)
  ret void
}

declare i32 @printf(ptr noundef, ...) #1

; Function Attrs: noinline nounwind uwtable
define internal void @_GLOBAL__sub_I_incr_module_1() #2 section ".text.startup" {
entry:
  call void @__stmts__0()
  ret void
}

attributes #0 = { "min-legal-vector-width"="0" }
attributes #1 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #2 = { noinline nounwind uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }

!llvm.module.flags = !{!0, !1, !2, !3, !4}
!llvm.ident = !{!5}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 8, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = !{i32 7, !"uwtable", i32 2}
!4 = !{i32 7, !"frame-pointer", i32 2}
!5 = !{!"clang version 22.0.0git (https://github.com/anutosh491/llvm-project.git 81ad8fbc2bb09bae61ed59316468011e4a42cf47)"}
=== end compile-ptu ===

execute-ptu 1: [TU=0x55556cfbf830, M=0x55556cfc13a0 (incr_module_1)]
hi
```

Basically I see that CodeGen emits IR for a cell before we know whether
DiagnosticsEngine has an error. For C code like `printf("hi\n");`
without <stdio.h>, Sema emits a diagnostic but still produces a
"codegen-able" `TopLevelStmt`, so the `printf` call is IR-generated into
the current module.

Previously, when `Diags.hasErrorOccurred()` was true, we only cleaned up
the PTU AST and left the CodeGen module untouched. The next successful
cell then called `GenModule()`, which returned that same module (now
also containing the next cell’s IR), causing side effects from the
failed cell (e.g. printf)