[PATCH] MC: Allow targets to stop symbol name quoting

[Rafael Espíndola]

>> This function is used to print assembly. You have an assembler that can
>> handle any characters in the name? How?
>
> No, but it also does not accept quoted symbols to avoid them. There is
> currently a suboptimal pass that handles a subset of the character
> restrictions HSAIL has which renames globals and replaces the disallowed
> characters with something else.
>
> The real problem with this is that the characters used for different kinds
> of symbol prefixes (% and &) are rejected, causing the disallowed quotes to
> be added. The character rules are unlike other assembly targets and needs to
> be specially handled in some way, so I need a way to avoid assumptions about
> the allowed characters like this.

So, there are two things at play in here:

* What characters can an assembler handle without quotes.
* Can an assembler handle quoted names.

We should probably make that explicit with

* A isValidUnquotedName(StringRef Name) virtual function.
* A SupportsQuotedNames bool variable.

The logic for printing a symbol then is

if (isValidUnquotedName(name))
  print the name
else if (SupportsQuotedNames)
  print the name in quotes
else
  show an error.

Cheers,
Rafael