[codegen/llvm] Emit DWARF debug information for LoweredFunc instances

[ajtulloch]

Emit DWARF debug information. This allows us to set breakpoints on these generated functions, inspect arguments, show reasonable names in AOT mode in profiling tools, etc. There's a subsequent perf PR upcoming from Bram which augments this with support for Linux perf's jit interface, which makes perf work in JIT scenarios.

The implementation is fairly straightforward - for every LoweredFunc, we emit DWARF information for the function, it's signature, and it's arguments. You can see an example query in:

(lldb) image lookup -r -n myadd1 -v
1 match found in /var/folders/bk/2wcywy612zx2882l0z06y_w1f8bjlv/T/tmpv5ie135z/mylib.so:
        Address: mylib.so[0x0000000000001270] (mylib.so.__TEXT.__text + 0)
        Summary: mylib.so`myadd1 at model.tvm
         Module: file = "/var/folders/bk/2wcywy612zx2882l0z06y_w1f8bjlv/T/tmpv5ie135z/mylib.so", arch = "x86_64"
    CompileUnit: id = {0xffffffff00000000}, file = "/tmp/model.tvm", language = "c"
       Function: id = {0xffffffff00000036}, name = "myadd1", range = [0x0000000114693270-0x00000001146935f6)
       FuncType: id = {0xffffffff00000036}, compiler_type = "int (char *, char *, int)"
         Blocks: id = {0xffffffff00000036}, range = [0x114693270-0x1146935f6)
      LineEntry: [0x0000000114693270-0x000000011469327b): /tmp/model.tvm
         Symbol: id = {0x00000004}, range = [0x0000000114693270-0x0000000114693600), name="myadd1"
       Variable: id = {0xffffffff00000054}, name = "arg1", type = "char *", location = rdi, decl =
       Variable: id = {0xffffffff00000061}, name = "arg2", type = "char *", location = rsi, decl =
       Variable: id = {0xffffffff0000006e}, name = "arg3", type = "int32", location = rdx, decl =

Some more screenshots from Bram:

[tqchen]

Thanks @ajtulloch please fix the CI error, and also tag a few more reviewers

[ajtulloch]

cc @yidawang, @tqchen, @jroesch, @eqy

[tqchen]

@ajtulloch I have retriggered the CI several times and the test consistently failed at the TOPI test, can you look into what happened?

[ajtulloch]

@tqchen sure will take a look.

[jroesch]

LGTM modulo CI

[tqchen]

@ajtulloch any leads?

[ajtulloch]

@tqchen I'll just tried emit it only for CodeGenCPU backends.

[tqchen]

Thanks @ajtulloch @jroesch , this PR is merged