[OpenCL] Fix type compatibility check and generic AS mangling.

1. Reimplemented conditional operator so that it checks
compatibility of unqualified pointees of the 2nd and
the 3rd operands (C99, OpenCL v2.0 6.5.15).

Define QualTypes compatibility for OpenCL as following:

   - corresponding types are compatible (C99 6.7.3)
   - CVR-qualifiers are equal (C99 6.7.3)
   - address spaces are equal (implementation defined)

2. Added generic address space to Itanium mangling.

Review: D30037

Patch by Dmitry Borisenkov!

llvm-svn: 297468