[OMPT] Fix assertion for OpenMP code generated with outdated compilers

For up-to-date compilers, this assertion is reasonable, but it breaks
compatibility with the typical compiler installed on most systems.
This patch changes the default value to what we had when there was no
compiler support. A warning about the outdated compiler is printed during
runtime, when this point is reached.

Differential Revision: https://reviews.llvm.org/D39890

llvm-svn: 317928