Fix unrepresentable enum for clang-cl unstable ABI

When using LIBCXX_ABI_UNSTABLE=YES, clang-cl gave the following warning:

P:\llvm_master\src\llvm\projects\libcxx\include\string(683,51):
warning: enumerator value is not representable in the underlying type
'int' [-Wmicrosoft-enum-value]

Fixed by switching from enums to static const size_type.

https://reviews.llvm.org/D35174

llvm-svn: 307751