[Clang][OpenMP] Allow loop-transformations with template parameters.

Clang would reject

    #pragma omp for
    #pragma omp tile sizes(P)
    for (int i = 0; i < 128; ++i) {}

where P is a template parameter, but the loop itself is not
template-dependent. Because P context-dependent, the TransformedStmt
cannot be generated and therefore is nullptr (until the template is
instantiated by TreeTransform). The OMPForDirective would still expect
the a loop is the dependent context and trigger an error.

Fix by introducing a NumGeneratedLoops field to OMPLoopTransformation.
This is used to distinguish the case where no TransformedStmt will be
generated at all (e.g. #pragma omp unroll full) and template
instantiation is needed. In the latter case, delay resolving the
iteration space like when the for-loop itself is template-dependent
until the template instatiation.

A more radical solution would always delay the iteration space analysis
until template instantiation, but would also break many test cases.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D111124

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