Added missing StandardConversionSequence initialization

This is the second msan failure where UserDefinedConversion does not initialize
its `Before` member as identity conversion.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199997 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] libclang: Update GENERATED_HEADERS with actually used.

+  ClangAttrVisitor
+  ClangCommentCommandList

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199999 91177308-0d34-0410-b5e6-96231b3b80d8