swift234

Re-apply "Reduce memory footprint of the Swift compiler"

Use malloc/free for allocating/freeing SIL instructions instead of using the BumpPtrAllocator. This allows for memory reuse and significantly reduces the memory footprint of the compiler.

For example, a peak memory usage during a compilation of the standard library and StdlibUnitTest is reduced by 25%-30%. The performance of the compiler seems to be not affected by this change, i.e. no slowdown is measured.

The use-after-free issues reported by build bots are fixed now.

rdar://23303031