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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@257384 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm