IR: Stop relying on GetStringMapEntryFromValue()

It relies on undefined behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223438 91177308-0d34-0410-b5e6-96231b3b80d8
ADT: Remove GetStringMapEntryFromValue()

It relies on undefined behaviour, since `StringMapEntry<>` is not
a standard layout type.  There are no users anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223439 91177308-0d34-0410-b5e6-96231b3b80d8