Change the syntax of the malloc and free commands in lldb-test's
ir-memory-map subcommand to:
<malloc> ::= <label> = malloc <size> <alignment> <free> ::= free <label>
This should make it easier to read and extend tests in the future, e.g
to test IRMemoryMap::WriteMemory or double-free behavior.
s/startswith/consumeFront. Then, you don't need to do the .find(' ') below.