Avoid referencing the vtable when calling the ctor without emitting it

This fixes compilation errors about incomplete types used with WebKit's
RefPtr template.  Simply calling an out of line constructor should not
instantiate all inline and defaulted virtual methods.

Tested by building and testing several big piles of code on Linux.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D4429

llvm-svn: 213109