[libc] Do not redefine wchar_t when compiling in C++ (llvm#110769)

Summary:
This is a fundamental type in C++, so we can't redefine it.

Author: jhuber6

libc/include/llvm-libc-types/wchar_t.h

@@ -9,6 +9,11 @@
 #ifndef LLVM_LIBC_TYPES_WCHAR_T_H
 #define LLVM_LIBC_TYPES_WCHAR_T_H
 
+// wchar_t is a fundamental type in C++.
+#ifndef __cplusplus
+
 typedef __WCHAR_TYPE__ wchar_t;
 
+#endif
+
 #endif // LLVM_LIBC_TYPES_WCHAR_T_H