[lld/mac] Add support for -flat_namespace
-flat_namespace makes lld emit binaries that use name lookup that's more in
line with other POSIX systems: Instead of looking up symbols as (dylib,name)
pairs by dyld, they're instead looked up just by name.
-flat_namespace has three effects:
- MH_TWOLEVEL and MH_NNOUNDEFS are no longer set in the Mach-O header
- All symbols use BIND_SPECIAL_DYLIB_FLAT_LOOKUP as ordinal
- When a dylib is added to the link, its dependent dylibs are also added, so that lld can verify that no undefined symbols remain at the end of a link with -flat_namespace. These transitive dylibs are added for symbol resolution, but they are not emitted in LC_LOAD_COMMANDs.
-undefined with -flat_namespace still isn't implemented. Before this change,
it was impossible to hit that combination because -flat_namespace caused a
diagnostic. Now that it no longer does, emit a dedicated temporary diagnostic
when both flags are used.
Differential Revision: https://reviews.llvm.org/D97641