In the ASTReader, replace the continuous range maps whose value types

In the ASTReader, replace the continuous range maps whose value types
were (Module*, Offset) with equivalent maps whose value type is just a
Module*. The offsets have moved into corresponding "Base" fields
within the Module itself, where they will also be helpful for
local->global translation (eventually).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136441 91177308-0d34-0410-b5e6-96231b3b80d8