[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.

llvm-svn: 302284

clang/include/clang/AST/ODRHash.h

@@ -25,7 +25,7 @@ namespace clang {
 
 class Decl;
 class IdentifierInfo;
-class NestedNameSpecifer;
+class NestedNameSpecifier;
 class Stmt;
 class TemplateParameterList;