Mangle based on the declaration we're given, not the canonical

Mangle based on the declaration we're given, not the canonical
declaration, since attributes that affect mangling may have been added
to subsequent declarations. However, to determine the linkage of the
declaration, we need to look at the canonical declaration. Fixes PR4412.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@85400 91177308-0d34-0410-b5e6-96231b3b80d8