Workaround a performance issue with modules + PCH

More fallout from r228234; when looking up an identifier in a PCH that
imports the Cocoa module on Darwin, it was taking 2 to 5 seconds
because we were hammering the MapVector::erase() function, which is
O(n).  For now, just clear() the contained SmallVector to get back to
0.25 - 0.5 seconds.  This is probably not the long-term fix, because
without modules or without PCH the performance is more like 0.02
seconds.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234655 91177308-0d34-0410-b5e6-96231b3b80d8