Implement caching for the linkage and visibility calculations of

declarations.

The motivation for this patch is that linkage/visibility computations
are linear in the number of redeclarations of an entity, and we've run
into a case where a single translation unit has > 6500 redeclarations
of the same (unused!) external variable. Since each redeclaration
involves a linkage check, the resulting quadratic behavior makes Clang
slow to a crawl. With this change, a simple test with 512
redeclarations of a variable syntax-checks ~20x faster than
before.

That said, I hate this change, and will probably end up reverting it
in a few hours. Reasons to hate it:
  - It makes NamedDecl larger, since we don't have enough free bits in
  Decl to squeeze in the extra information about caching.
  - There are way too many places where we need to invalidate this
  cache, because the visibility of a declaration can change due to
  redeclarations (!). Despite self-hosting and passing the testsuite,
  I have no confidence that I've found all of places where this cache
  needs to be invalidated.

llvm-svn: 120808