[WIP] Associate messages with failures in debug info

[jckarter]

Based on @adrian-prantl's idea. Improve the experience of debugging Swift traps by encoding a reason message in the line table as the name of a fake inlined function, so that debuggers and DWARF-aware symbolicators like addr2line present the message in the backtrace, for example:

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x0000000100001494 foo`main [inlined] freedom at foo.swift:45:1 [opt]
   42  	  Builtin.condfail_message(xcondition.value, message.value)
   43  	}
   44  	
-> 45  	myAssert(2 + 2 != 4, "too much freedom")
Target 0: (foo) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
  * frame #0: 0x0000000100001494 foo`main [inlined] too much freedom at foo.swift:45:1 [opt]
    frame #1: 0x0000000100001494 foo`main at foo.swift:45 [opt]
    frame #2: 0x00007fff72fddc49 libdyld.dylib`start + 1
    frame #3: 0x00007fff72fddc49 libdyld.dylib`start + 1

TODO:

• Should we mark up the inlined function scope with some sort of flag so that debuggers know it's a trap reason?
• For this to work in -Onone, we'd need a mandatory pass that cleans up struct_extract (struct) and inttoptr (ptrtoint) to be able to look through the abstraction of String and StaticString in transparent code.
• It would be interesting to also be able to support interpolating frame variable values in the message. Using something similar to what @ravikandhadai has done with os_log, we could potentially encode interpolated messages like fatalError("foo \(x)") in debug info by encoding x as a local variable of the inlined trap function location, allowing the debugger to try to find and present the value as part of the failure message.

[jckarter]

@compnerd It sounds like CodeView debug info has some additional invariants that aren't clear to me. I tried to preserve the existing logic here, but how would what we're trying to do here fit in with Windows debug info?

[jckarter]

It looks like Erik already started on this as #25978.