[StackProtector] Fix computation of GSCookieOffset and EHCookieOffset with SEH4

Summary:
Fix the computation of the offsets present in the scopetable when using the
SEH (__except_handler4).

This patch added an intrinsic to track the position of the allocation on the
stack of the EHGuard. This position is needed when producing the ScopeTable.

```
    struct _EH4_SCOPETABLE {
        DWORD GSCookieOffset;
        DWORD GSCookieXOROffset;
        DWORD EHCookieOffset;
        DWORD EHCookieXOROffset;
        _EH4_SCOPETABLE_RECORD ScopeRecord[1];
    };

    struct _EH4_SCOPETABLE_RECORD {
        DWORD EnclosingLevel;
        long (*FilterFunc)();
            union {
            void (*HandlerAddress)();
            void (*FinallyFunc)();
        };
    };
```

The code to generate the EHCookie is added in `X86WinEHState.cpp`.
Which is adding these instructions when using SEH4.

```
Lfunc_begin0:
# BB#0:                                 # %entry
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ebx
	pushl	%edi
	pushl	%esi
	subl	$28, %esp
	movl	%ebp, %eax                <<-- Loading FramePtr
	movl	%esp, -36(%ebp)
	movl	$-2, -16(%ebp)
	movl	$L__ehtable$use_except_handler4_ssp, %ecx
	xorl	___security_cookie, %ecx
	movl	%ecx, -20(%ebp)
	xorl	___security_cookie, %eax  <<-- XOR FramePtr and Cookie
	movl	%eax, -40(%ebp)           <<-- Storing EHGuard
	leal	-28(%ebp), %eax
	movl	$__except_handler4, -24(%ebp)
	movl	%fs:0, %ecx
	movl	%ecx, -28(%ebp)
	movl	%eax, %fs:0
	movl	$0, -16(%ebp)
	calll	_may_throw_or_crash
LBB1_1:                                 # %cont
	movl	-28(%ebp), %eax
	movl	%eax, %fs:0
	addl	$28, %esp
	popl	%esi
	popl	%edi
	popl	%ebx
	popl	%ebp
	retl

```

And the corresponding offset is computed:
```
Luse_except_handler4_ssp$parent_frame_offset = -36
	.p2align	2
L__ehtable$use_except_handler4_ssp:
	.long	-2                      # GSCookieOffset
	.long	0                       # GSCookieXOROffset
	.long	-40                     # EHCookieOffset    <<----
	.long	0                       # EHCookieXOROffset
	.long	-2                      # ToState
	.long	_catchall_filt          # FilterFunction
	.long	LBB1_2                  # ExceptionHandler

```

Clang is not yet producing function using SEH4, but it's a work in progress.
This patch is a step toward having a valid implementation of SEH4.
Unfortunately, it is not yet fully working. The EH registration block is not
allocated at the right offset on the stack.

Reviewers: rnk, majnemer

Subscribers: llvm-commits, chrisha

Differential Revision: http://reviews.llvm.org/D21231

llvm-svn: 273281