[OpenMP] Fix support for device as host

Without this patch, when the offload device is set to
`omp_get_initial_device()`, the runtime fails with an error diagnostic
when entering target regions or target data regions.

However, OpenMP 5.1, sec. 2.14.5 "target Construct", "Restrictions",
p. 203, L3-5 states:

> The device clause expression must evaluate to a non-negative integer
> value that is less than or equal to the value of
> omp_get_num_devices().

Sec. 3.7.7 "omp_get_initial_device", p. 412, L2-3 states:

> The value of the device number is the value returned by the
> omp_get_num_devices routine.

Similarly, OpenMP 5.0, sec. 2.12.5 "target Construct", "Restrictions",
p. 174 L30-32 states:

> The device clause expression must evaluate to a non-negative integer
> value less than the value of omp_get_num_devices() or to the value
> of omp_get_initial_device().

This patch fixes this behavior by changing the runtime to behave as if
offloading is disabled whenever it finds the offload device (either
from a `device` clause or the default device) is set to the host
device.  In the case of mandatory offloading when
`omp_get_num_devices() == 0`, it incorporates the behavior proposed
for OpenMP 5.2 in OpenMP spec github issue 2669.

Reviewed By: grokos, RaviNarayanaswamy

Differential Revision: https://reviews.llvm.org/D97616

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