Explictly track tentative definitions within Sema, then hand those

Explictly track tentative definitions within Sema, then hand those
tentative definitions off to the ASTConsumer at the end of the
translation unit.

Eliminate CodeGen's internal tracking of tentative definitions, and
instead hook into ASTConsumer::CompleteTentativeDefinition. Also,
tweak the definition-deferal logic for C++, where there are no
tentative definitions.

Fixes <rdar://problem/6808352>, and will make it much easier for
precompiled headers to cope with tentative definitions in the future.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69681 91177308-0d34-0410-b5e6-96231b3b80d8