[PATCHES] Enhance standard DWARF for Ada

[Pierre-Marie de Rodat <derodat@adacore.com>]

Hello,

This patch series aims at enhancing GCC to emit standard DWARF in place 
of the current GNAT encodings (non-standard DWARF) for a set of "basic" 
types: dynamic arrays, variable-length records, variant parts, etc.

As most debuggers (including GDB) do not support these DWARF 
descriptions yet, it is necessary to still make it possible to generate 
the old GNAT encodings, though. Hence the first change, which introduces 
an option (-fgnat-encodings=all|gdb|minimal) that selects how much GNAT 
encodings must be emitted:

   - -fgnat-encodings=all emits as much encodings as before (i.e. before 
these series of patches);

   - -fgnat-encodings=gdb (the default) emits all the standard DWARF 
that the current GDB can handle (so this will vary with time) and emit 
GNAT encodings for the rest;

   - -fgnat-encodings=minimal emits as much standard DWARF as possible 
in the compiler and still emit GNAT encodings for the rest (this will 
vary with time as well).

At the moment, there is no difference between -fgnat-encodings=all|gdb 
but I hope this will change soon thanks to ongoing work on GDB (for 
instance: 
<https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=df25ebbd091aebc132f97ffd6ce9cf7964a57981>).

The remaining commits enhance the DWARF back-end (mostly dwarf2out.c), 
the Ada front-end (mostly gcc/ada/gcc-interface/*) and sometimes adapts 
the Fortran front-end as well in order to emit this new DWARF. In order 
to do so, it sometimes introduces new language hooks to pass information 
that aren't available in trees.

The result has been bootstrapped and regtested successfully on 
x86_64-linux. Besides, this triggers no regression by default 
(-fgnat-encodings=gdb). Note however that being unable to test 
-fgnat-encodings=minimal under with debugger right now, I also have 
written a small Python testsuite using pyelftools[1] in order to check 
for a few examples that the expected DIEs/attributes are properly 
generated (that was useful during development but is very ad-hoc, so I 
haven't published it yet).

Ok for trunk? Thank you in advance for the review!

[1] https://github.com/eliben/pyelftools/

-- 
Pierre-Marie de Rodat