[PATCH] Some infrastructure work for virtual file system (now on phab)

[Rafael Espíndola]

> Look at CompilerInstance. It has a number of IntrusiveRefCntPtr’s that exist because we consistently ran into problems with other clients needing to hold onto these core, cooperating objects for longer than the given CompilerInstance. ASTUnit is one of the major culprits here, but it’s not the only one.

Yes. I am not suggesting for CompilerInstance to own the FS. That
would be really odd.

>> But is it not the case that
>> * A clang process will always use a single FS.
>> * The decision as to which to use can happen just after parsing the
>> command line arguments?
>
>
> We may spawn a number of Clang invocations within the same thread to build modules from headers, and they all need to share the same FS.

Which perfectly matches to having a FS being constructed before any
CompilerInstance and each CompilerInstance has a pointer to the same
FS. In other words, a FS is a singleton in a clang process, no?

Cheers,
Rafael