[CUDA] Allow function overloads in CUDA based on host/device attributes.

The patch makes it possible to parse CUDA files that contain host/device
functions with identical signatures, but different attributes without
having to physically split source into host-only and device-only parts.

This change is needed in order to parse CUDA header files that have
a lot of name clashes with standard include files.

Gory details are in design doc here: https://goo.gl/EXnymm
Feel free to leave comments there or in this review thread.

This feature is controlled with CC1 option -fcuda-target-overloads
and is disabled by default.

Differential Revision: http://reviews.llvm.org/D12453

llvm-svn: 248295