Comment parsing: add CommentOptions to allow specifying custom comment block commands

Add an ability to specify custom documentation block comment commands via a new
class CommentOptions.  The intention is that this class will hold future
customizations for comment parsing, including defining documentation comments
with specific numbers of parameters, etc.

CommentOptions instance is a member of LangOptions.

CommentOptions is controlled by a new command-line parameter
-fcomment-block-commands=Foo,Bar,Baz.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175892 91177308-0d34-0410-b5e6-96231b3b80d8