[SEH] Re-land r234532, but use internal linkage for all SEH helpers

Even though these symbols are in a comdat group, the Microsoft linker
really wants them to have internal linkage.

I'm planning to tweak the mangling in a follow-up change. This is a
straight revert with a 1-line fix.

llvm-svn: 234613

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -41,7 +41,7 @@ CodeGenFunction::CodeGenFunction(CodeGenModule &cgm, bool suppressNewContext)
       CurFn(nullptr), CapturedStmtInfo(nullptr),
       SanOpts(CGM.getLangOpts().Sanitize), IsSanitizerScope(false),
       CurFuncIsThunk(false), AutoreleaseResult(false), SawAsmBlock(false),
-      BlockInfo(nullptr), BlockPointer(nullptr),
+      IsOutlinedSEHHelper(false), BlockInfo(nullptr), BlockPointer(nullptr),
       LambdaThisCaptureField(nullptr), NormalCleanupDest(nullptr),
       NextCleanupDestIndex(1), FirstBlockInfo(nullptr), EHResumeBlock(nullptr),
       ExceptionSlot(nullptr), EHSelectorSlot(nullptr),