X86: permit using SjLj EH on x86 targets as an option

This adds support to the backed to actually support SjLj EH as an exception
model.  This is *NOT* the default model, and requires explicitly opting into it
from the frontend.  GCC supports this model and for MinGW can still be enabled
via the `--using-sjlj-exceptions` options.

Addresses PR27749!

llvm-svn: 271244
diff --git a/llvm/lib/CodeGen/LLVMTargetMachine.cpp b/llvm/lib/CodeGen/LLVMTargetMachine.cpp
index 346555a..b164cad 100644
--- a/llvm/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/llvm/lib/CodeGen/LLVMTargetMachine.cpp
@@ -75,6 +75,9 @@
 
   TmpAsmInfo->setRelaxELFRelocations(Options.RelaxELFRelocations);
 
+  if (Options.ExceptionModel != ExceptionHandling::None)
+    TmpAsmInfo->setExceptionsType(Options.ExceptionModel);
+
   AsmInfo = TmpAsmInfo;
 }