[libcxx] Support providing symbol suffix through compiler define

When -funique-internal-linkage-names option is enabled, Clang will
append a unique suffix `.uniq.<MD5 hash of the source file>` to all
internal symbols. Since we cannot detect when this option is enabled or
compute the source file hash from within the source file itself, the
unique suffix needs to be supplied by the build system. The
`_LIBCPP_SYMBOL_SUFFIX` can be used for that purpose.

Author: petrhosek

libcxx/src/include/overridable_function.h

@@ -56,6 +56,15 @@
 // with this macro must be defined at global scope.
 //
 
+// When -funique-internal-linkage-names option is enabled, Clang will append a unique suffix
+// `.uniq.<MD5 hash of the source file>` to all internal symbols. Since we cannot detect when
+// this option is enabled or compute the source file hash from within the source file itself,
+// the unique suffix needs to be supplied by the build system. The `_LIBCPP_SYMBOL_SUFFIX` can
+// be used for that purpose.
+#ifndef _LIBCPP_SYMBOL_SUFFIX
+#define _LIBCPP_SYMBOL_SUFFIX
+#endif
+
 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -68,8 +77,8 @@ _LIBCPP_END_NAMESPACE_STD
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
     static __attribute__((used)) type symbol##_impl__ arglist __asm__("_" _LIBCPP_TOSTRING(symbol));                   \
-    __asm__(".globl _" _LIBCPP_TOSTRING(symbol));                                                                      \
-    __asm__(".weak_definition _" _LIBCPP_TOSTRING(symbol));                                                            \
+    __asm__(".globl _" _LIBCPP_TOSTRING(symbol) _LIBCPP_SYMBOL_SUFFIX);                                                \
+    __asm__(".weak_definition _" _LIBCPP_TOSTRING(symbol) _LIBCPP_SYMBOL_SUFFIX);                                      \
     extern __typeof(symbol##_impl__) name __attribute__((weak_import));                                                \
     _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
     template <>                                                                                                        \
@@ -91,7 +100,7 @@ _LIBCPP_END_NAMESPACE_STD
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
     static type symbol##_impl__ arglist __asm__(_LIBCPP_TOSTRING(symbol##_impl__));                                    \
-    [[gnu::weak, gnu::alias(_LIBCPP_TOSTRING(symbol##_impl__))]] type name arglist;                                    \
+    [[gnu::weak, gnu::alias(_LIBCPP_TOSTRING(symbol##_impl__) _LIBCPP_SYMBOL_SUFFIX)]] type name arglist;              \
     _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
     template <>                                                                                                        \
     inline bool __is_function_overridden<static_cast<type(*) arglist>(name)>() {                                       \