clover: Add a function for retrieving a device's preferred ir v3

clover: Add a function for retrieving a device's preferred ir v3

A device now has two function for getting information about the IR
it needs to return.

ir_format() => returns the preferred IR
ir_target() => returns the triple for the target that is understood by
                 clang/llvm.

v2:
  - renamed ir_target() to ir_format()
  - renamed llvm_triple() to ir_target()

v3:
  - Remove unnecessary include
  - Do proper conversion from std::vector<char> to std::string

Reviewed-by: Francisco Jerez <currojerez@riseup.net>