[OpenMP][CodeGen] Add codegen for combined 'loop' directives.

The loop directive is a descriptive construct which allows the compiler
flexibility in how it generates code for the directive's associated
loop(s). See OpenMP specification 5.2 [257:8-9].

Codegen added in this patch for the combined 'loop' directives are:

'target teams loop'     -> 'target teams distribute parallel for'
'teams loop'            -> 'teams distribute parallel for'
'target parallel loop'  -> 'target parallel for'
'parallel loop'         -> 'parallel for'

NOTE: The implementation of the 'loop' directive itself is unchanged.

Differential Revision: https://reviews.llvm.org/D145823