The semantic spelling enumeration should retain values to the spelling list indexes...

The semantic spelling enumeration should retain values to the spelling list indexes used by the attribute. The only attribute affected by this in practice is the OpenCLImageAccessAttr, which has duplicate semantic spellings that are automatically stripped.

We do not implicitly create an OpenCLImageAccessAttr, so this change only affects out of tree users. There is no way to test this behavior specifically that I can see, since this only affects implicit creation of attributes.

Fixes PR22403.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231803 91177308-0d34-0410-b5e6-96231b3b80d8