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.
