When deduplicating unreachable blocks, erase the source information.

After deduplication the block conceptually belongs to multiple locations
in the source. Although these blocks are unreachable, in #123341 we did
come across a real side effect, an unreachable block that survives into
the compiled code can cause a debugger to set a breakpoint on the wrong
instruction. Erasing the source information ensures that a debugger will
never be misled into thinking that the unreachable block is worth setting
a breakpoint on, especially after #128627.

Technically we don't need to erase the source information if all the
deduplicated blocks have identical source information, but tracking
that seems like more effort than it's worth.

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