[flang][openacc] parse and ignore non-standard shortloop clause (#106564)

shortloop is a non standard OpenACC extension
(https://docs.nvidia.com/hpc-sdk/pgi-compilers/2015/pgirn157.pdf) that
can be found on loop directives.

f18 parser was choking when seeing it. Since it can be found in existing
apps and is mainly an optimization hint, parse it on loop directives and
ignore it with a warning.

For the records, here is shortloop meaning according to the manual linked above:

"If the shortloop clause appears on a loop directive with the vector clause, it tells the compiler that the
loop trip count is less than or equal to the number of vector lanes created for that loop. This means the
value of the vector() clause on the loop directive in a kernels region, or the value of the
vector_length() clause on the parallel directive in a parallel region will be greater than or
equal to the loop trip count. This allows the compiler to generate more efficient code for the loop"

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