Skip type units/type uniquing when we know we're only emitting the type once (vtable-based emission when triggered by a strong vtable, with -fno-standalone-debug)

(this would regress size without a corresponding LLVM change that avoids
putting other user defined types inside type units when they aren't in
their own type units - instead emitting declarations inside the type
unit and a definition in the primary CU)

Reviewers: aprantl

Differential Revision: https://reviews.llvm.org/D61079

llvm-svn: 359235