Introduce the local -> global declaration ID mapping into the AST

Introduce the local -> global declaration ID mapping into the AST
reader, to allow AST files to be loaded with their declarations
remapped to different ID numbers. Fix a number of places where we were
either failing to map local declaration IDs into global declaration
IDs or where interpreting the local declaration IDs within the wrong
module.

I've tested this via the usual "random gaps" method. It works well
except for the preamble tests, because our handling of the precompiled
preamble requires declaration and preprocessed entity to be stable
when parsing code and then loading that back into memory. This
property will hold in general, but my randomized testing naturally
breaks this property to get more coverage. In the future, I expect
that the precompiled preamble logic won't need this property.

I am very unhappy with the current handling of the translation unit,
which is a rather egregious hack. We're going to have to do something
very different here for loading multiple AST files, because we don't
want to have to cope with merging two translation units. Likely, we'll
just handle translation units entirely via "update" records, and
predefine a single, fixed declaration ID for the translation
unit. That will come later.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136779 91177308-0d34-0410-b5e6-96231b3b80d8