[Modules] When writing out the on-disk hash table for the decl context

[Modules] When writing out the on-disk hash table for the decl context
lookup tables, we need to establish a stable ordering for constructing
the hash table. This is trickier than it might seem.

Most of these cases are easily handled by sorting the lookup results
associated with a specific name that has an identifier. However for
constructors and conversion functions, the story is more complicated.
Here we need to merge all of the constructors or conversion functions
together and this merge needs to be stable. We don't have any stable
ordering for either constructors or conversion functions as both would
require a stable ordering across types.

Instead, when we have constructors or conversion functions in the
results, we reconstruct a stable order by walking the decl context in
lexical order and merging them in the order their particular declaration
names are encountered. This doesn't generalize as there might be found
declaration names which don't actually occur within the lexical context,
but for constructors and conversion functions it is safe. It does
require loading the entire decl context if necessary to establish the
ordering but there doesn't seem to be a meaningful way around that.

Many thanks to Richard for talking through all of the design choices
here. While I wrote the code, he guided all the actual decisions about
how to establish the order of things.

No test case yet because the test case I have doesn't pass yet -- there
are still more sources of non-determinism. However, this is complex
enough that I wanted it to go into its own commit in case it causes some
unforseen issue or needs to be reverted.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@233156 91177308-0d34-0410-b5e6-96231b3b80d8