[CMake] Detect information about test compiler

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

llvm-svn: 319448