Include the correct conversion context locations for condition expressions.

This improves the conversion diagnostics (by correctly pointing to the loop
construct for conversions that may've been caused by the contextual conversion
to bool caused by a condition expression) and also causes the NULL conversion
warnings to be correctly suppressed when crossing a macro boundary in such a
context. (previously, since the conversion context location was incorrect, the
suppression could not be performed)

Reported by Nico Weber as feedback to r156826.

llvm-svn: 156901