[Support] Use HAVE_DLOPEN to guard dlopen(3) usage

Summary:
The usage was previously guarded by HAVE_DLFCN. This breaks on Android with
LLVM_BUILD_STATIC as the platform does not provide a static version of libdl.
Using HAVE_DLOPEN fixes it as the code will only get used if we are actually able
to link an executable using dlopen.

Reviewers: rafael, beanz

Subscribers: tberghammer, danalbert, llvm-commits

Differential Revision: https://reviews.llvm.org/D26504

llvm-svn: 288246