Commit 9a8d477a0e0 ("[OpenCL] Add builtin function attribute
handling", 2019-11-05) stopped Clang from mangling single-overload
builtins, which is incorrect.
Add handling for the "pure", "const" and "convergent" function
attributes for OpenCL builtin functions.
Patch by Pierre Gondois and Sven van Haastregt.
Differential Revision: https://reviews.llvm.org/D64319