Tag build swift-DEVELOPMENT-SNAPSHOT-2016-09-20-a
Merge remote-tracking branch 'origin/swift-3.1-branch' into stable

* origin/swift-3.1-branch:
  [docs] Touch up the code coverage doc
  [Driver] Report invalid -mtune/-mcpu parameters when -arch=arm64
  [Coverage] Suppress creating a code region if the same area is covered by an expansion region.
  [test] Pass a fake libLTO.dylib to a driver test which depends on it
  [Coverage] Prevent creating a redundant counter if a nested body ends with a macro.
  [AST] Make InitListExpr::isExplicit const (NFC)
  [AST] Remove unused function, to silence a GCC7 warning.