[libclang] Use same USR encoding for 'class' as 'struct'.

[libclang] Use same USR encoding for 'class' as 'struct'.

'class' and 'struct' can be used interchangebly for forward references.
Use the same encoding otherwise we may get into a weird situation where the USR for the same
declaration is different based on whether the definition of the tag reference is visible or not.

llvm-svn: 223632