Keep track of all of the class and function template's "common"

Keep track of all of the class and function template's "common"
pointers in the ASTContext, so that the folding sets stored inside
them will be deallocated when the ASTContext is destroyed (under
-disable-free). <rdar://problem/7998824>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@104465 91177308-0d34-0410-b5e6-96231b3b80d8