[modules] The key to a DeclContext name lookup table is not actually a

[modules] The key to a DeclContext name lookup table is not actually a
DeclarationName (because all ctor names are considered the same, and so on).
Reflect this in the type used as the lookup table key. As a side-effect, remove
one copy of the duplicated code used to compute the hash of the key.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246124 91177308-0d34-0410-b5e6-96231b3b80d8