Force honoring nthreads-var and thread-limit-var inside teams construct on host

This patch fixes https://bugs.llvm.org/show_bug.cgi?id=42906, via adding
adjustment of number of threads on enter to the teams construct on host
according to user settings. This allows to pass checks and avoid assertions
at time of team of threads creation.

Patch by Andrey Churbanov

Differential Revision: https://reviews.llvm.org/D66351

llvm-svn: 369430