swift234

StringLiteralExpr: always include a good source range

When we are interpreting escape sequences in the lexer, we copy the string
literal bytes to ASTContext instead of storing a pointer to the source buffer.
But then we used to try to get a source location for that string in the heap,
which is not a valid source buffer. It succeeds during parsing, but breaks
when we try to print a diagnostic using this location.

Added a verifier check for this.

Also added a real source range for StringLiteralExpr, instead of a source range
with begin == end, produced from the beginning location.


Swift SVN r5961