[lldb] Stop a cached read at an invalid range (#218997)
`MemoryCache::Read` refused a read only when its first byte sat in a
range
the process had recorded as invalid. A read whose interior met such a
range
was served in full, out of a cache line fetched straight across it,
which is
what the `FIXME` in that function described:
```
// FIXME: We should do a more thorough check to make sure that we're not
// overlapping with any invalid ranges (e.g. Read 0x100 - 0x200 but there's an
// invalid range 0x180 - 0x280).
```
With an invalid range at `[0xE010, 0xE020)`, a 64-byte read at `0xE000`
returned 64 bytes and a success status.
Test the whole requested range for an intersection instead. Report the
failure, and shorten the read to the bytes below the range, so a caller
gets
the readable prefix and an error rather than data the process said is
not
there. A read starting inside a range still serves nothing.
