[modules] Incrementally compute the list of overridden module macros based on

[modules] Incrementally compute the list of overridden module macros based on
the active module macros at the point of definition, rather than reconstructing
it from the macro history. No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235941 91177308-0d34-0410-b5e6-96231b3b80d8