[libc++] Fix unrepresentable enum for clang-cl unstable ABI

[bcraig]

Summary

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.

[EricWF]

Maybe using a static const variable instead of an enum would work better? __long_mask should never actually be ODR used so a definition for it should never actually be needed.

[bcraig]

Switched to static consts

[EricWF]

LGTM.

include/string

#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
     struct __long
     {
         pointer   __data_;
         size_type __size_;
         size_type __cap_;
     };
 
 #if _LIBCPP_BIG_ENDIAN
-    enum {__short_mask = 0x01};
-    enum {__long_mask  = 0x1ul};
+    static const size_type __short_mask = 0x01;
+    static const size_type __long_mask  = 0x1ul;
 #else  // _LIBCPP_BIG_ENDIAN
-    enum {__short_mask = 0x80};
-    enum {__long_mask  = ~(size_type(~0) >> 1)};
+    static const size_type __short_mask = 0x80;
+    static const size_type __long_mask  = ~(size_type(~0) >> 1);
 #endif  // _LIBCPP_BIG_ENDIAN
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                       (sizeof(__long) - 1)/sizeof(value_type) : 2};
 
     struct __short
     {
         value_type __data_[__min_cap];
#else
     struct __long
     {
         size_type __cap_;
         size_type __size_;
         pointer   __data_;
     };
 
 #if _LIBCPP_BIG_ENDIAN
-    enum {__short_mask = 0x80};
-    enum {__long_mask  = ~(size_type(~0) >> 1)};
+    static const size_type __short_mask = 0x80;
+    static const size_type __long_mask  = ~(size_type(~0) >> 1);
 #else  // _LIBCPP_BIG_ENDIAN
-    enum {__short_mask = 0x01};
-    enum {__long_mask  = 0x1ul};
+    static const size_type __short_mask = 0x01;
+    static const size_type __long_mask  = 0x1ul;
 #endif  // _LIBCPP_BIG_ENDIAN
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                       (sizeof(__long) - 1)/sizeof(value_type) : 2};
 
     struct __short
     {
         union