Fix build error with gcc in swift demangling code
Fixes:
third_party/swift/include/swift/Demangling/Demangler.h:228:11:
error: declaration of ‘swift::Demangle::NodeFactory::Slab* swift::Demangle::NodeFactory::Checkpoint::Slab’
changes meaning of ‘Slab’ [-fpermissive]
228 | Slab *Slab;
| ^~~~
third_party/swift/include/swift/Demangling/Demangler.h:47:10:
note: ‘Slab’ declared here as ‘struct swift::Demangle::NodeFactory::Slab’
47 | struct Slab {
| ^~~~
No intended behavior change.
