simplify interaction between new JITs and codegen

[vtjnash]

this treats the new JITs (MCJIT, ORCJIT) much like the old JIT, but using Module as the atomic unit instead of Function

reviewer note: much of the code motion is moving the logic to interact with the JIT into jitlayers.cpp. this is not yet separable from codegen.cpp, but I expect they should be able to divide into separate compilation units in the future.

TODO:

• reimplement memory optimization of strings in the JIT
• reimplement code-coverage and memory-usage counters

fix #15533

[Keno]

if (f)?

[Keno]

I'm generally fine with this, even though it goes in the opposite direction from where I was taking it. I do worry about the performance implications of creating this many modules, but since we're at least not doing per-module codegen, it might not be that bad. As I mentioned, I would like to see some of this code move into ORCJIT proper eventually, but I'm fine with leaving it like this if it makes compatibility with the old JITs easier.

[vtjnash]

bump. this is ready to merge.

[vtjnash]

breadcumb: in the future, we may want to just call Linker::linkModules (introduced in llvm-3.8 for LTO), but for now, our actual usage is much easier than that because we don't need full generality (or barely any at all), so we can just do the simple thing here

[josefsachsconning]

I think this broke the JULIA_THREADS=1 build, since the number of arguments to jl_get_specialization1 was changed. Here is the bottom of my make output.

    CC src/threading.o
/home/sachs/src/julia-multithreading/src/threading.c: In function ‘jl_threading_run’:
/home/sachs/src/julia-multithreading/src/threading.c:394:5: error: too many arguments to function ‘jl_get_specialization1’
     jl_lambda_info_t *li = jl_get_specialization1(argtypes, NULL);
     ^
In file included from /home/sachs/src/julia-multithreading/src/threading.c:23:0:
/home/sachs/src/julia-multithreading/src/julia_internal.h:266:19: note: declared here
 jl_lambda_info_t *jl_get_specialization1(jl_tupletype_t *types);
                   ^
make[1]: *** [threading.o] Error 1
make: *** [julia-src-release] Error 2