Added a per-process cache for reserved memory

regions that aren't actually allocated in the
process.  This cache is used by the expression
parser if the underlying process doesn't support
memory allocation, to avoid needless repeated
searches for unused address ranges.

Also fixed a silly bug in IRMemoryMap where it
would continue searching even after it found a
valid region.

<rdar://problem/13866629>

llvm-svn: 182028