Revert r70075 and r70078, which reorganized the PCH on-disk hash table

Revert r70075 and r70078, which reorganized the PCH on-disk hash table
for identifiers to separate "interesting" from "uninteresting"
identifiers. However, to cope with compiler invocations where the
predefines buffers mismatch, we need to be able to search the complete
identifier table. Cocoa.h.pch is now about 500k larger that it used to
be :(

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70320 91177308-0d34-0410-b5e6-96231b3b80d8