Zig allows multiple extern functions with the same name, and the
backends have to handle this possibility.
For LLVM, we keep a sparse map of collisions, and then resolve them in
flushModule(). This introduces some technical debt that will have to be
resolved when adding incremental compilation support to the LLVM
backend.
My previous commit added a new behavior test that passes for stage2 but
I forgot to check whether it passes for stage1. Since it does not, it
has to be disabled.
Additionally, this commit organizes behavior tests; there is no longer a
section of tests only passing for stage1. Instead, tests are disabled on
an individual basis. There is an except for the file which has global
assembly in it.
