Perform a nested CMake invocation to avoid writing our own parser for compiler versions when we are not testing the in-tree compiler. Use the extracted information to mark a test as unsupported that hangs with Clang prior to version 4.0.1 and restrict tests for libomptarget to Clang version 6.0.0 and later. Differential Revision: https://reviews.llvm.org/D40083 git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@319448 91177308-0d34-0410-b5e6-96231b3b80d8