[Docs][llvm-mc] Add documentation on --filetype flag

[aidengrossman]

Summary

Currently the filetype flag is not documented, and knowing the behavior
of this flag is fairly important for doing anything other than
disassembling to text assembly.

[aidengrossman]

I've also noticed that when doing assembly without setting a --filetype, it still defaults to asm, which doesn't make a lot of sense. There should probably be some input validation to ensure that the various modes (eg --assemble and --disassemble) are run with the correct filetypes and automatically provide sensible defaults, since outputting text assembly by default when trying to assemble text assembly doesn't make a lot of sense. I'm planning on opening another patch to fix this behavior.

[lattner]

nice

llvm/docs/CommandGuide/llvm-mc.rst

  Parse a series of hex bytes, and print the result out as assembly syntax.
 
 
 
 .. option:: --mdis
 
  Marked up disassembly of string of hex bytes.
 
+.. option:: --filetype=[asm,null,obj]
+
+ Sets the output filetype. Setting this flag to `asm` will make the tool
+ output text assembly. Setting this flag to `obj` will make the tool output
+ an object file. Setting it to `null` causes no output to be created and can be
+ used for timing purposes. The default value is `asm`.
+
 .. option:: -g
 
  Generate DWARF debugging info for assembly source files.
 
 .. option:: --large-code-model
 
  Create CFI directives that assume the code might be more than 2 GB.