[CIR] __builtin_source_location lowering for constant (#194505)
This handles the UnnamedGlobalConstantDecl lowering, which is only used
for the source-location object. This uses similar logic to the classic
compiler, so we end up with roughly the same IR.
There is a slight difference in how we choose which strings to uniquify
(preexisting) and how we name them/name duplicates, but it isn't really
relevant to the text itself.