SILBoxTypes have their own generic signature and substitution
map. This means that every time we query isEscapable or mayEscape, we
need to extract the type of the box's field and perform type
substitution so that the AST query only sees types from the function's
generic environment.
Fixes rdar://124179106 (Assertion failed in SIL:
(!type->hasTypeParameter() && "caller forgot to mapTypeIntoContext!"))
Optionally, the dependency to the initialization of the global can be specified with a dependency token `depends_on <token>`.
This is usually a `builtin "once"` which calls the initializer for the global variable.
By default it lowers the builtin to an `alloc_vector` with a paired `dealloc_stack`.
If the builtin appears in the initializer of a global variable and the vector elements are initialized,
a statically initialized global is created where the initializer is a `vector` instruction.
