[ODRHash] Fix typo, NFC

NestedNameSpecifer to NestedNameSpecifier.  This was not a problem before since
one of the included headers transitively brought in the definition of the class
and only manifested as a problem when using the typoed NestedNameSpecifer and
getting an incomplete type error instead of a typo correction.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302284 91177308-0d34-0410-b5e6-96231b3b80d8