Epic: implement flow-sensitive bounds declarations

[dtarditi]

The bounds widening for pointers to null-terminated arrays relies on flow-sensitive bounds declarations for widening bounds using strlen. Currently the bounds widening is unsound because the flow-sensitive bounds declarations are not checked or used. We should implement flow-sensitive bounds checking. The implementation can be done in 4 stages:

• Compute the extent of bounds declarations: Compute the extent of flow-sensitive bounds declarations #1208
• Check bounds declarations in _Where clauses: Check bounds declarations in where clauses #1209
• Use the flow-sensitive bounds declarations for bounds-checking memory accesses: Use flow-sensitive bounds declarations for bounds-checking memory accesses #1210.
• Check flow-sensitive bounds declarations in their extents. We also have to make sure that statements within the extent of bounds declaration maintain the flow-sensitive bounds declarations: Check flow-sensitive bounds declarations within their extents #1211