Use the range machinery for DW_AT_ranges and DW_AT_high/lo_pc.

This commit moves us from a single range per subprogram to extending
ranges if we are:

a) In the same section, and
b) In the same enclosing CU.

This means we have more fine grained ranges for compile units, and fewer
ranges overall when we have multiple functions in the same CU
adjacent to each other in the object file.

Also remove all of the earlier hacks around this functionality for
function sections etc. Also update all of the testcases to take into
account the merging functionality.