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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257384 91177308-0d34-0410-b5e6-96231b3b80d8