Avoid multiple -Wunreachable-code diagnostics that are triggered by

the same source range and use the unary operator fixit only when it
actually silences the warning.

rdar://24570531

Differential Revision: https://reviews.llvm.org/D28231


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291757 91177308-0d34-0410-b5e6-96231b3b80d8