Simplify __kmp_expand_threads

This change simplifies __kmp_expand_threads to take a single argument.
Previously, it allowed two arguments and had logic to decide on different
potential expansion sizes. However, no calls to __kmp_expand_threads in the
runtime make use of this extra logic. Thus the extra argument and logic is
removed here.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D41836

llvm-svn: 322204