[DebugInfo][SILGen] Ensure "guard let" is lowered with a non-implicit location
Prior to this patch, a "guard let" was being lowered with an implicit debug
location, causing it to be dropped in later stages of the compiler, and making
it impossible to set a breakpoint in that line.
This was tracked down to a piece of code in `SILGenFunction::emitExprInto`,
which takes an optional Location parameter that was being ignored in one code
path.