Improve -Wunreachable-code to provide a means to indicate code is intentionally marked dead via if((0)).

Taking a hint from -Wparentheses, use an extra '()' as a sigil that
a dead condition is intentionally dead.  For example:

  if ((0)) { dead }

When this sigil is found, do not emit a dead code warning.  When the
analysis sees:

  if (0)

it suggests inserting '()' as a Fix-It.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205069 91177308-0d34-0410-b5e6-96231b3b80d8