[modules] Rearrange how redeclaration chains are loaded, to remove a walk over

all modules and reduce the number of declarations we load when loading a
redeclaration chain.

The new approach is:
 * when loading the first declaration of an entity within a module file, we
   first load all declarations of the entity that were imported into that
   module file, and then load all the other declarations of that entity from
   that module file and build a suitable decl chain from them
 * when loading any other declaration of an entity, we first load the first
   declaration from the same module file

As before, we complete redecl chains through name lookup where necessary.

To make this work, I also had to change the way that template specializations
are stored -- it no longer suffices to track only canonical specializations; we
now emit all "first local" declarations when emitting a list of specializations
for a template.

On one testcase with several thousand imported module files, this reduces the
total runtime by 72%.

llvm-svn: 245779

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