Revert "[Windows] Simplify assertion code. NFC."

This reverts commit r254363.

load64BitDebugHelp() has the side effect of loading dbghelp and setting
globals. It should be called in no-asserts builds as well as debug
builds.

llvm_unreachable is also not appropriate here, since we actually want to
return if dbghelp couldn't be loaded in a non-asserts build.

llvm-svn: 257384