[BOLT][NFC] Forward declare ReorderBlocks for MSVC19

Fix bolt-x86_64-wine-msvc builder:
https://lab.llvm.org/buildbot/#/builders/222/builds/1154

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D127612

bolt/lib/Passes/BinaryPasses.cpp

@@ -147,6 +147,9 @@ static cl::opt<bool>
                     cl::desc("dump CFG of functions with unknown control flow"),
                     cl::cat(BoltCategory), cl::ReallyHidden);
 
+// Please MSVC19 with a forward declaration: otherwise it reports an error about
+// an undeclared variable inside a callback.
+extern cl::opt<bolt::ReorderBasicBlocks::LayoutType> ReorderBlocks;
 cl::opt<bolt::ReorderBasicBlocks::LayoutType> ReorderBlocks(
     "reorder-blocks", cl::desc("change layout of basic blocks in a function"),
     cl::init(bolt::ReorderBasicBlocks::LT_NONE),