CodeGenModule.cpp: [PR33810][Modules] Remove an assertion that confirms MangledDeclNames[CanonicalGD] might be still empty.

FIXME: It is accepted that MangledDeclNames[CanonicalGD] is overwritten here?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309504 91177308-0d34-0410-b5e6-96231b3b80d8