[dsymutil] Add function size to the debug map.

The debug map embedded by ld64 in binaries conatins function sizes.
These sizes are less precise than the ones given by the debug information
(byte granularity vs linker atom granularity), but they might cover code
that is referenced in the line table but not in the DIE tree (that might
very well be a compiler bug that I need to investigate later).
Anyway, extracting that information is necessary to be able to mimic
dsymutil's behavior exactly.

llvm-svn: 232300