GetExprRange() (used by -Wconversion checking) should look through OpaqueValueExprs.

Fixes a false positive with -Wconversion involving Objective-C properties.

Fixes <rdar://problem/14415662>.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192611 91177308-0d34-0410-b5e6-96231b3b80d8