DebugLoc: Remove getFromDILexicalBlock()

The only user of `DebugLoc::getFromDILexicalBlock()` was creating a new
`MDLocation` as convenient API for passing an `MDScope`.  Stop doing
that, and remove the API.  If in the future we actually *want* to create
new DebugLocs, calling `MDLexicalBlock::get()` makes more sense.

llvm-svn: 233643