[YAMLTraits] Fix mapping <none> value that followed by comments.

When mapping an optional value, if the value is <none> and followed
by comments, there will be a parsing error. This patch helps fix this
issue.

e.g.,

When mapping the following YAML,

```
Sections:
  - Name:  blah
    Type:  SHT_foo
    Flags: [[FLAGS=<none>]] ## some comments.
```

the raw value of `ScalarNode` is "<none> " rather than "<none>". We need
to remove the spaces.

Differential Revision: https://reviews.llvm.org/D85180

Author: higuoxing

llvm/include/llvm/Support/YAMLTraits.h

@@ -1629,7 +1629,9 @@ void IO::processKeyWithDefault(const char *Key, Optional<T> &Val,
     bool IsNone = false;
     if (!outputting())
       if (auto *Node = dyn_cast<ScalarNode>(((Input *)this)->getCurrentNode()))
-        IsNone = Node->getRawValue() == "<none>";
+        // We use rtrim to ignore possible white spaces that might exist when a
+        // comment is present on the same line.
+        IsNone = Node->getRawValue().rtrim(' ') == "<none>";
 
     if (IsNone)
       Val = DefaultValue;

llvm/test/tools/yaml2obj/ELF/none-value.yaml

@@ -21,6 +21,7 @@ FileHeader:
 Sections:
   - Name:         .bar
     Type:         SHT_PROGBITS
+    Flags:        [[TEST=<none>]] ## Comment
     Offset:       [[TEST=<none>]]
     Address:      [[TEST=<none>]]
     Content:      [[TEST=<none>]]