Merge pending instantiations instead of overwriting existing ones.

Merge pending instantiations instead of overwriting existing ones.
Check whether a pending instantiation needs to be instantiated (or whether an instantiation already exists).
Verify the size of the PendingInstantiations record (was only checking size of existing PendingInstantiations).

Migrate Obj-C++ part of redecl-merge into separate test, now that this is growing.
templates.mm: test that CodeGen has seen exactly one definition of template instantiations.
redecl-merge.m: use "@" specifier for expected-diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164993 91177308-0d34-0410-b5e6-96231b3b80d8