This enables additional tests for the ClangImporter. This found a
missing piece in the `-enable-objc-interop` work that was done
previously. Address that and enable the tests. There are now the
following failing tests on Linux:
* sdk - depends on Foundation (not hermetic, see SR-7572)
* mixed-nsuinteger - depends on Foundation (not hermetic, see SR-7572)
* import-mixed-with-header-twice - requires apple/swift PR#16022
* can_import_objc_idempotent - requires apple/swift PR#16022
* objc_protocol_renaming - requires apple/swift PR#16022
This includes 'LONG_MAX' in the Darwin module, which is defined by the
system and then immediately redefined by Clang's copy of limits.h.
The general strategy here is that if two definitions of a macro in the
same module are in separate explicit submodules, we need to keep track
of both of them, but if they're in the same explicit submodule then
one is probably deliberately redefining the other. This isn't fully
correct because one explicit submodule could include another explicit
submodule, but it's a good first approximation, which we can refine if
we come across problem cases in practice.
Swift /also/ has the ability to distinguish between ModuleA.MACRO and
ModuleB.MACRO if you've imported both modules, although there's an
issue that the two of them end up getting the same mangled name if you
try to use both in the same compilation unit. (Filed
rdar://problem/34968281.) That ability is preserved with this change.
All of this will likely change if/when we switch to Clang's "local
submodule visibility" mode, which is needed for the C++ Modules TS but
is also incompatible with Apple's SDKs at the moment. In that case,
macros in two separate explicit submodules will no longer have an
'override' relationship just because they're mentioned sequentially in
the module map.
rdar://problem/34413934
