Patch by GM: apparently '__value' (two underscores) is a special name in Visual Studio, so rename the private method in <regex> with that name. GM's patch used  '___value' (three underscores), but I changed that to '__regex_traits_value' because I've been burned in the past by identifiers that appear identical but are not.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@193087 91177308-0d34-0410-b5e6-96231b3b80d8