[modules] In C++, stop serializing and deserializing a list of declarations in

[modules] In C++, stop serializing and deserializing a list of declarations in
the identifier table. This is redundant, since the TU-scope lookups are also
serialized as part of the TU DeclContext, and wasteful in a number of ways. We
still emit the decls for PCH / preamble builds, since for those we want
identical results, not merely semantically equivalent ones.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242855 91177308-0d34-0410-b5e6-96231b3b80d8