MCDwarf: Refactor line table handling into a single data structure

This replaces several "compile unit ID -> thing" mappings in favor of
one mapping from CUID to the whole line table structure (files,
directories, and lines).

This is another step along the way to refactoring out reusable
components of line table handling for use when generating debug_line.dwo
for fission type units.

Also, might be a good basis to fold some of this handling down into
MCStreamers to avoid the special case of "One line table when doing asm
printing, line table per CU otherwise" by building it into the different
MCStreamer implementations.
[analyzer] Warn when passing pointers to const but uninitialized memory.

Passing a pointer to an uninitialized memory buffer is normally okay,
but if the function is declared to take a pointer-to-const then it's
very unlikely it will be modifying the buffer. In this case the analyzer
should warn that there will likely be a read of uninitialized memory.

This doesn't check all elements of an array, only the first one.
It also doesn't yet check Objective-C methods, only C functions and
C++ methods.

This is controlled by a new check: alpha.core.CallAndMessageUnInitRefArg.

Patch by Per Viberg!