[modules] Stop performing PCM lookups for all identifiers when building with C++ modules. Instead, serialize a list of interesting identifiers and mark those ones out of date on module import. Avoiding the identifier lookups here gives a 20-30% speedup in builds with large numbers of modules. No functionality change intended.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242868 91177308-0d34-0410-b5e6-96231b3b80d8