driver: Support checking for rlimits via cmake (when bootstrapping)

Summary:
Add a cmake check for sys/resource.h and replace the __has_include() check with its result, in order to make it possible to use rlimits when building with compilers not supporting __has_include() -- i.e. when bootstrapping.

// Please also re-apply dfcd52eb1d8e5d322404b40414cb7331c7380a8c (llvm-config.h fix)

Patch by: Michał Górny

Reviewers: rsmith, beanz

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23744

llvm-svn: 279559