Rename feature test for lambda init-captures from cxx_generalized_capture to
cxx_init_capture. "generalized" is neither descriptive nor future-proof. No
compatibility problems expected, since we've never advertised having this
feature.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187058 91177308-0d34-0410-b5e6-96231b3b80d8