[GWP_ASAN] Avoid using VERSION_GREATER_EQUAL in cmake files

This is a fixup for r369823 which introduced the use of
VERSION_GREATER_EQUAL in the cmake config for gwp_asan.

Minimum supported version of cmake in LLVM is 3.4.3 and
VERSION_GREATER_EQUAL was not introduced until later
versions of cmake.

llvm-svn: 369891