MC,test: Improve section group test

Also add a case for #146581
```
.section sec,"ax"
.section .foo,"axG",@progbits,sec
nop
```

Author: MaskRay

llvm/test/MC/ELF/comdat.s

@@ -0,0 +1,59 @@
+# RUN: llvm-mc -filetype=obj -triple x86_64 %s -o %t
+# RUN: llvm-readelf -SsX %t | FileCheck %s
+# RUN: llvm-readelf -g %t | FileCheck %s --check-prefix=GROUP
+
+## Test that we produce the group sections and that they are before the members
+
+# CHECK:       [ 3] .group            GROUP           0000000000000000 {{.*}} 00000c 04     14   1  4
+# CHECK-NEXT:  [ 4] .foo              PROGBITS        0000000000000000 {{.*}} 000001 00 AXG  0   0  1
+# CHECK-NEXT:  [ 5] .bar              PROGBITS        0000000000000000 {{.*}} 000001 00 AXG  0   0  1
+# CHECK-NEXT:  [ 6] .group            GROUP           0000000000000000 {{.*}} 000008 04     14   2  4
+# CHECK-NEXT:  [ 7] .zed              PROGBITS        0000000000000000 {{.*}} 000001 00 AXG  0   0  1
+# CHECK-NEXT:  [ 8] .group            GROUP           0000000000000000 {{.*}} 00000c 04     14   4  4
+# CHECK-NEXT:  [ 9] .baz              PROGBITS        0000000000000000 {{.*}} 000004 00 AXG  0   0  1
+# CHECK-NEXT:  [10] .rela.baz         RELA            0000000000000000 {{.*}} 000018 18   G 14   9  8
+# CHECK-NEXT:  [11] sec               PROGBITS        0000000000000000 {{.*}} 000000 00      0   0  1
+# CHECK-NEXT:  [12] .group            GROUP           0000000000000000 {{.*}} 000008 04     14   3  4
+# CHECK-NEXT:  [13] .qux              PROGBITS        0000000000000000 {{.*}} 000000 00 AXG  0   0  1
+# CHECK-NEXT:  [14] .symtab           SYMTAB
+
+## Test that g1 and g2 are local, but g3 is an undefined global.
+
+# CHECK:      0000000000000000     0 NOTYPE  LOCAL  DEFAULT    4 (.foo)    g1
+# CHECK-NEXT: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    6 (.group)  g2
+# CHECK-NEXT: 0000000000000000     0 SECTION LOCAL  DEFAULT   11 (sec)     sec
+# CHECK-NEXT: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND           g3
+
+# GROUP:       COMDAT group section [    3] `.group' [g1] contains 2 sections:
+# GROUP-NEXT:     [Index]    Name
+# GROUP-NEXT:     [    4]   .foo
+# GROUP-NEXT:     [    5]   .bar
+# GROUP-EMPTY:
+# GROUP-NEXT:  COMDAT group section [    6] `.group' [g2] contains 1 sections:
+# GROUP-NEXT:     [Index]    Name
+# GROUP-NEXT:     [    7]   .zed
+# GROUP-EMPTY:
+# GROUP-NEXT:  COMDAT group section [    8] `.group' [g3] contains 2 sections:
+# GROUP-NEXT:     [Index]    Name
+# GROUP-NEXT:     [    9]   .baz
+# GROUP-NEXT:     [   10]   .rela.baz
+# GROUP-EMPTY:
+# GROUP-NEXT:  group section [   12] `.group' [] contains 1 sections:
+# GROUP-NEXT:     [Index]    Name
+# GROUP-NEXT:     [   13]   .qux
+
+	.section	.foo,"axG",@progbits,g1,comdat
+g1:
+        nop
+
+        .section	.bar,"ax?",@progbits
+        nop
+
+        .section	.zed,"axG",@progbits,g2,comdat
+        nop
+
+        .section	.baz,"axG",@progbits,g3,comdat
+        .long g3
+
+.section sec
+.section .qux,"axG",@progbits,sec