[IRMemoryMap] Use labels in the "malloc" and "free" lldb-test commands

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.

Differential Revision: https://reviews.llvm.org/D47646

llvm-svn: 333930