Projects like libc use mutually exclusive macros to compile files
multiple times and then merge the result into the final library. For
installapi to accept these, we'd need to parse the same declarations in
different ways. This patch adds the basic pipelining for installapi to
create the correct TBD file.
* -Xproject allows: -fmodules, -fobjc-arc, fvisibility=hidden, prefix
headers
* -Xlabel allows: -D and -U settings
* Error on 'private' and 'public' labels -X<label>
* Xplatform allows: -iframework <path> This is to support the case where
zippered frameworks want to pass in iOSSupport search path.
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