Change representation of instruction ranges where variable is accessible.

Change representation of instruction ranges where variable is accessible.

Use more straightforward way to represent the set of instruction
ranges where the location of a user variable is defined - vector of pairs
of instructions (defining start/end of each range),
instead of a flattened vector of instructions where some instructions
are supposed to start the range, and the rest are supposed to "clobber" it.

Simplify the code which generates actual .debug_loc entries.

No functionality change.

llvm-svn: 209698