[IRMemoryMap] Fix the alignment adjustment in Malloc

This prevents Malloc from allocating the same chunk of memory twice, as
a byproduct of an alignment adjustment which gave the client access to
unallocated memory.

Prior to this patch, the newly-added test failed with:

$ lldb-test ir-memory-map ... ir-memory-map-overlap1.test
...
Command: malloc(size=64, alignment=32)
Malloc: address = 0x1000cd080
Command: malloc(size=64, alignment=8)
Malloc: address = 0x1000cd0b0
Malloc error: overlapping allocation detected, previous allocation at [0x1000cd080, 0x1000cd0c0)

Differential Revision: https://reviews.llvm.org/D47551