Now MICache is a linked list (per the FIXME), where we tradeoff between MacroInfo objects being in MICache
and MIChainHead.  MacroInfo objects in the MICache chain are already "Destroy()'ed", so they can be reused.  When
inserting into MICache, we need to remove them from the regular linked list so that they aren't destroyed more than
once.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116869 91177308-0d34-0410-b5e6-96231b3b80d8