[llvm] [llvm-readobj] enable demangle option for the xcoff object file (PR #78455)

[James Henderson via llvm-commits]

================
@@ -61,6 +61,11 @@ file formats.
   Dump decompressed section content when used with ``-x`` or ``-p``.
   If the section(s) are not compressed, they are displayed as is.
 
+.. option:: --demangle, -C
+
+ Display demangled symbol names in the output; the option only is for ELF and
+ XCOFF file format.
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jh7370 wrote:

```suggestion
 Display demangled symbol names in the output. This option is only for ELF and
 XCOFF file formats.
```
Sorry, I missed this before.

Also, the option is still present in the ELF-specific section of this document. Please delete it from there.

https://github.com/llvm/llvm-project/pull/78455