Add caching to the MemoryMappingLayout class on Linux. This is necessary for...

Add caching to the MemoryMappingLayout class on Linux. This is necessary for the cases when a sandbox prevents ASan from reading the mappings
from /proc/self/maps.
The mappings are currently being cached on each access to /proc/self/maps. In the future we'll need to add an API that allows the client to notify ASan about the sandbox.

llvm-svn: 169076