If, while loading a bridging header, we pick up a Clang module that
claims to have an overlay Swift module, and that Swift module turns
out to have a bridging header, we can end up reallocating the array
of modules to process while we're looping over it. Be defensive
against this occurrence.
This just fixes a crash; it does not at all solve the problem of this
being broken in several ways:
- Accidentally naming your module the same as a system module shadows
the latter (if the system module is a Swift module) or *makes your
module into an overlay* (if the system module is a Clang module).
- Bridging headers are only officially supported on executable targets
and unit tests, but this isn't really enforced.
- Implicit inclusion of a bridging header *when you import a Swift
module* is a hack to begin with, and a hack that worsens when the
main module also has a bridging header. (All the bridging headers
get folded together into the "same" module, which leads to more
visibility than desired as well as cycles in the import graph.)
- Combining all of these can result in some pretty bizarre behavior.
rdar://problem/54581756
