Migrate the mish-mash of declaration checks in

Migrate the mish-mash of declaration checks in
Sema::ActOnUninitializedDecl over to InitializationSequence (with
default initialization), eliminating redundancy. More importantly, we
now check that a const definition in C++ has an initilizer, which was
an #if 0'd code for many, many months. A few other tweaks were needed
to get everything working again:

  - Fix all of the places in the testsuite where we defined const
    objects without initializers (now that we diagnose this issue)
  - Teach instantiation of static data members to find the previous
    declaration, so that we build proper redeclaration
    chains. Previously, we had the redeclaration chain but built it
    too late to be useful, because...
  - Teach instantiation of static data member definitions not to try
    to check an initializer if a previous declaration already had an
    initializer. This makes sure that we don't complain about static
    const data members with in-class initializers and out-of-line
    definitions.
  - Move all of the incomplete-type checking logic out of
    Sema::FinalizeDeclaratorGroup; it makes more sense in
    ActOnUnitializedDecl.

There may still be a few places where we can improve these
diagnostics. I'll address that as a separate commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95657 91177308-0d34-0410-b5e6-96231b3b80d8