PR19698, PR19674: enable __has_feature checks for cxx_generic_lambdas and

cxx_decltype_auto, and fix documentation of cxx_generic_lambdas and
cxx_init_captures to specify the right feature-check name.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208445 91177308-0d34-0410-b5e6-96231b3b80d8