Print the SIL function body on an assert. Recovering the SIL code is the
critical path for pretty much any SIL development. The only alternative is
rebuilding the library with string matching or building a debug compiler and
hoping lldb works. The standard library takes a very long time to build with a
debug compiler.
remove the mixed concept that was SILFileLocation.
Also add support for a third type of underlying storage that will be used
for deserialized debug lcoations from textual SIL.
NFC
<rdar://problem/22706994>
