[OpenMP][CodeGen] Improved codegen for combined loop directives (#87278)

IR for 'target teams loop' is now dependent on suitability of associated
loop-nest.

If a loop-nest:

- does not contain a function call, or
- the -fopenmp-assume-no-nested-parallelism has been specified,
- or the call is to an OpenMP API AND
- does not contain nested loop bind(parallel) directives

then it can be emitted as 'target teams distribute parallel for', which
is the current default. Otherwise, it is emitted as 'target teams
distribute'.

Added debug output indicating how 'target teams loop' was emitted. Flag
is -mllvm -debug-only=target-teams-loop-codegen

Added LIT tests explicitly verifying 'target teams loop' emitted as a
parallel loop and a distribute loop.

Updated other 'loop' related tests as needed to reflect change in IR.
- These updates account for most of the changed files and
additions/deletions.