[Clang] Implement the rest of __builtin_elementwise_* functions.

The patch implement the rest of __builtin_elementwise_* functions
specified in D111529, including:
* __builtin_elementwise_floor
* __builtin_elementwise_roundeven
* __builtin_elementwise_trunc

Signed-off-by: Jun <jun@junz.org>

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D115429