Make Driver::link and LinkingContext::validate return true on success.

This patch inverts the return value of these functions, so that they return
"true" on success and "false" on failure. The meaning of boolean return value
was mixed in LLD; for example, InputGraph::validate() returns true on success.
With this patch they'll become consistent.

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1748