[libc++] Include "bits/alltypes.h" to provide mbstate_t when using mu…

…sl libc

With D148542, we ran into the following libc++ build error when using musl libc.

```
.../musl/include/bits/alltypes.h:354:16:
error: definition of type '__mbstate_t' conflicts with typedef of the same name
typedef struct __mbstate_t { unsigned __opaque1, __opaque2; } mbstate_t;
               ^
.../sysroot/usr/include/bits/types/__mbstate_t.h:21:3: note: '__mbstate_t' declared here
} __mbstate_t;
  ^
1 error generated.
```
This is because the mbstate_t definition in musl libc conflicts with the one
from "bits/types/mbstate_t.h", and this patch attempts to fix this build issue
when musl libc is used.

Reviewed By: iana

Differential Revision: https://reviews.llvm.org/D151740

libcxx/include/__mbstate_t.h

@@ -35,7 +35,11 @@
 #  define __CORRECT_ISO_CPP_WCHAR_H_PROTO
 #endif
 
-#if __has_include(<bits/types/mbstate_t.h>)
+#if defined(_LIBCPP_HAS_MUSL_LIBC)
+#   define __NEED_mbstate_t
+#   include <bits/alltypes.h>
+#   undef __NEED_mbstate_t
+#elif __has_include(<bits/types/mbstate_t.h>)
 #   include <bits/types/mbstate_t.h> // works on most Unixes
 #elif __has_include(<sys/_types/_mbstate_t.h>)
 #   include <sys/_types/_mbstate_t.h> // works on Darwin