Tighten code.  No functionality change.
NSErrorChecker: remove quoting the parameter name in the diagnostic until we actually include it's name.

This is a possible regression of moving to using ImplicitNullDerefEvent.
Fixing this for real (including the parameter name) requires more
plumbing in ImplicitNullDerefEvent.  This is just a stop gap fix.
Require set-xcode-analyzer to run with Python 2.7 or later.

Addresses LLVM PR 11661.
Add __has_feature support to detect if clang supports the explicit "atomic" keyword for ObjC properties.

Fixes <rdar://problem/12953378>.
Update CMakeLists.txt