Fix cpu-dispatch MV regression caused by r347812

Fix cpu-dispatch MV regression caused by r347812

r347812 permitted forward declarations for cpu-dispatch functions, which
are occassionally useful as exposition in  header files.  However, this inadvertently
permitted this function to become multiversioned after a usage.  This
patch ensures that the "CausesMV" checks are still run in the
forward-declaration case.

Change-Id: Icb6f975a2d068f088b89e3bbe26cf1d24f5a972c

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@351212 91177308-0d34-0410-b5e6-96231b3b80d8