Implement P1766R1: diagnose giving non-C-compatible classes a typedef name for linkage purposes.
Due to a recent (but retroactive) C++ rule change, only sufficiently
C-compatible classes are permitted to be given a typedef name for
linkage purposes. Add an enabled-by-default warning for these cases, and
rephrase our existing error for the case where we encounter the typedef
name for linkage after we've already computed and used a wrong linkage
in terms of the new rule.
Differential Revision: https://reviews.llvm.org/D74103