[yaml2obj][ELF] Make symbol table top-level key.

Although in reality the symbol table in ELF resides in a section, the
standard requires that there be no more than one SHT_SYMTAB. To enforce
this constraint, it is cleaner to group all the symbols under a
top-level `Symbols` key on the object file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184627 91177308-0d34-0410-b5e6-96231b3b80d8

test/Object/yaml2obj-elf-symbol-LocalGlobalWeak.yaml

@@ -10,21 +10,19 @@ Sections:
     Type: SHT_PROGBITS
     Flags: [ SHF_ALLOC, SHF_WRITE ]
     Content: "DEADBEEF"
-  - Name: .symtab
-    Type: SHT_SYMTAB
-    Symbols:
-      Local:
-        - Name: local_symbol
-          Type: STT_OBJECT
-          Section: .data
-      Global:
-        - Name: global_symbol
-          Type: STT_OBJECT
-          Section: .data
-      Weak:
-        - Name: weak_symbol
-          Type: STT_OBJECT
-          Section: .data
+Symbols:
+  Local:
+    - Name: local_symbol
+      Type: STT_OBJECT
+      Section: .data
+  Global:
+    - Name: global_symbol
+      Type: STT_OBJECT
+      Section: .data
+  Weak:
+    - Name: weak_symbol
+      Type: STT_OBJECT
+      Section: .data
 
 # CHECK: Symbol {
 # CHECK:   Name:  (0)

test/Object/yaml2obj-elf-symbol-basic.yaml

@@ -16,16 +16,14 @@ Sections:
                       # This YAML file is a valid relocatable object that,
                       # when linked and run on x86_64, will go into an
                       # infloop.
-  - Name: .symtab
-    Type: SHT_SYMTAB
-    Symbols:
-      Global:
-        - Name: main
-          Type: STT_FUNC
-          Section: .text
-          Value: 0x1
-          Size: 2
-        - Name: undefined_symbol
+Symbols:
+  Global:
+    - Name: main
+      Type: STT_FUNC
+      Section: .text
+      Value: 0x1
+      Size: 2
+    - Name: undefined_symbol
 
 # CHECK:      Symbols [
 # CHECK-NEXT:   Symbol {