We already zero init AllocStack, and here's the same.
The debugger's variable view shows up variables on the line
where they're declared (before they've been initialized).
In some cases, we just print garbage. In some others, it's
dangerous (imagine an array which we believe has 2^32 elements
because we ended up reusing a stack slot). This way it's always
consistent, as lldb uses the first word to understand whether
an object is initialized or not here.
Fixes <rdar://problem/39883298>
To avoid the debugger displaying garbage or having expressions crash
when inspecting an uninitialized dictionary variable, zero-initialize
the first word of the alloca at -Onone.
<rdar://problem/36619561>
This un-breaks the LLDB testsuite.
To make it unambiguous whether a `var` binding has been initialized,
zero-initialize the first pointer-sized field. LLDB uses this to
recognize to detect uninitizialized variables. This can be removed once
swiftc switches to @llvm.dbg.addr() intrinsics. This dead store will get
optimized away when optimizations are enabled.
<rdar://problem/36156857>
