Fix/Improve SourceRange of explicitly defaulted members

When adding the implicit compound statement (required for Codegen?), the
end location was previously overridden by the start location, probably
based on the assumptions:

* The location of the compound statement should be the member's location
* The compound statement if present is the last element of a FunctionDecl

This patch changes the location of the compound statement to the
member's end location.

Code review: http://reviews.llvm.org/D4175