[lldb] Add --all option to "memory region"

This adds an option to the memory region command
to print all regions at once. Like you can do by
starting at address 0 and repeating the command
manually.

memory region [-a] [<address-expression>]

(lldb) memory region --all
[0x0000000000000000-0x0000000000400000) ---
[0x0000000000400000-0x0000000000401000) r-x <...>/a.out PT_LOAD[0]
<...>
[0x0000fffffffdf000-0x0001000000000000) rw- [stack]
[0x0001000000000000-0xffffffffffffffff) ---

The output matches exactly what you'd get from
repeating the command. Including that it shows
unmapped areas between the mapped regions.

(this is why Process GetMemoryRegions is not
used, that skips unmapped areas)

Help text has been updated to show that you can have
an address or --all but not both.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D111791