[clang][IFS][test] GreenDragon and Fuchsia Darwin bot fix: BindArchClass Nest.

On Darwin the clang driver does not invoke Driver::BuildActions directly
due to the need to handle Universal apps. Because of this there is a
difference in code path where Driver::BuildUniversalActions is called
instead of Driver::BuildActions. BuildUniversalActions ends up calling
Driver::BuildAction but what it does differently is it takes the driver
actions returned and wraps them each into a BindArchAction.

In Driver::BuildJobs there is a check for '-o' to determine that
multiple files are not specified when passing -o, except for Clang
Interface Stub this need to be an exception as we actually want to write
out multiple files: for every libfoo.so we have a libfoo.ifso sidecar
ifso file, etc. To allow this to happen there is a check for
IfsMergeAction, which is permitted to write out a secondary file. Except
on Darwin, the IfsMergeAction gets wrapped in the BindArchAction by
Driver::BuildUniversalActions so the check fails.

This patch is to look inside a BindArchAction in Driver::BuildJobs to
determine if there is in fact an IfsMergeAction, and if-so (pun intended)
allow the secondary sidecard ifs/ifso file to be written out.