Update to Excecption Handling(llvm.eh.sjlj.setjmp) documentation and undo accidental changes.

Author: anoopkg6

clang/lib/CodeGen/CGBuiltin.cpp

@@ -4621,6 +4621,8 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
 
     if (getTarget().getTriple().getArch() == llvm::Triple::systemz) {
       // On this target, the back end fills in the context buffer completely.
+      // It doesn't really matter if the frontend stores to the buffer before
+      // calling setjmp, the back-end is going to overwrite them anyway.
       Function *F = CGM.getIntrinsic(Intrinsic::eh_sjlj_setjmp);
       return RValue::get(Builder.CreateCall(F, Buf.emitRawPointer(*this)));
     }

llvm/docs/ExceptionHandling.rst

@@ -374,10 +374,11 @@ overall functioning of this intrinsic is compatible with the GCC
 to interoperate.
 
 The single parameter is a pointer to a five word buffer in which the calling
-context is saved. The front end places the frame pointer in the first word, and
-the target implementation of this intrinsic should place the destination address
-for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are
-available for use in a target-specific manner.
+context is saved. The format and contents of the buffer are target-specific.
+On certain targets, the front end places the frame pointer in the first word
+and the stack pointer in the third word, while the target implementation of
+this intrinsic fills in the remaining words.  On other targets, saving the
+calling context to the buffer is left completely to the target implementation. 
 
 .. _llvm.eh.sjlj.longjmp:
 

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -1,4 +1,4 @@
-//===-- systemzisellowering.cpp - systemz dag lowering implementation -----===//
+//===-- SystemZISelLowering.cpp - SystemZ DAG lowering implementation -----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.