[Sanitizers] Fix read buffer overrun in scanning loader commands

The fix only affects Darwin, but to write the test I had to modify
the MemoryMappingLayout class which is used by all OSes,
to allow for mocking of image header (this change should be NFC). Hence no [Darwin] in the subject
so I can get more eyes on it.

While looking for a memory gap to put the shadow area into, the sanitizer code
scans through the loaded images, and for each image it scans through its
loader command to determine the occupied memory ranges.

While doing so, if the 'segment load' (kLCSegment) loader comand is encountered, the command scanning function
returns success (true), but does not decrement the command list iterator counter.
The result is that the function is called again and again, with the iterator counter
now being too high. The command scanner keeps updating the loader command pointer,
by using the command size field.

If the loop counter is too high, the command pointer
lands into unintended area ( beyond <header ...