[libc++] Fix modules build - Rework __refstring definition

Summary:
`__libcpp_refstring` currently has two different definitions. First there is the complete definition in `<__refstring>` but there is also a second in  `<stdexcept>`.  The historical reason for this split is because both libc++ and libc++abi need to see the inline definitions of __libcpp_refstrings methods, but the `<stdexcept>` header doesn't.  However this is an ODR violation and breaks the modules build.

This patch fixes the issue by creating a single class definition in `<stdexcept>` and changing `<__refstring>` to contain only the inline method definitions. This way both `libcxx/src/stdexcept.cpp` and `libcxxabi/src/stdexcept.cpp` see the same declaration in `<stdexcept>` and definitions in `<__refstring>`

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25603

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285100 91177308-0d34-0410-b5e6-96231b3b80d8

include/stdexcept

@@ -53,17 +53,23 @@ public:
 #pragma GCC system_header
 #endif
 
-#ifndef _LIBCPP___REFSTRING
 _LIBCPP_BEGIN_NAMESPACE_STD
-class _LIBCPP_HIDDEN __libcpp_refstring {
-#ifdef __clang__
-    const char *__imp_ __attribute__((__unused__)); // only clang emits a warning
-#else
-    const char *__imp_;
-#endif
+
+class _LIBCPP_HIDDEN __libcpp_refstring
+{
+    const char* __imp_;
+
+    bool __uses_refcount() const;
+public:
+    explicit __libcpp_refstring(const char* msg);
+    __libcpp_refstring(const __libcpp_refstring& s) _NOEXCEPT;
+    __libcpp_refstring& operator=(const __libcpp_refstring& s) _NOEXCEPT;
+    ~__libcpp_refstring();
+
+    const char* c_str() const _NOEXCEPT {return __imp_;}
 };
+
 _LIBCPP_END_NAMESPACE_STD
-#endif
 
 namespace std  // purposefully not using versioning namespace
 {