Don't change the selected platform when creating the dummy target.

A dummy target is used by TargetList::CreateTargetInternal to prime newly
created targets. the first time this is done it creates the dummy target. The
dummy target is created with the host platform (See
TargetList::CreateDummyTarget) which results in switching the selected platform
back to the host platform even when creating a target for a different platform.
This change avoids changing the selected platform while creating the dummy
target to prevent this side effect.

Test Plan:
./dotest.py $DOTEST_OPTS -t -p TestCreateAfterAttach.py
Tests using process attach (e.g. TestCreateAfterAttach.py, TestHelloWorld.py)
now run successfully mac -> linux.

Differential Revision: http://reviews.llvm.org/D9712

llvm-svn: 237221