Runtimes: resync from branch (NFCI)

This restores the `LLVM_NODISCARD` macro from the branch point
(retaining the header inclusion changes). This is required to build
SwiftRemoteMirror without injecting another LLVM build.

Author: compnerd

stdlib/include/llvm/Support/Compiler.h

@@ -142,6 +142,23 @@
 #define LLVM_ATTRIBUTE_USED
 #endif
 
+/// LLVM_NODISCARD - Warn if a type or return value is discarded.
+
+// Use the 'nodiscard' attribute in C++17 or newer mode.
+#if defined(__cplusplus) && __cplusplus > 201402L && LLVM_HAS_CPP_ATTRIBUTE(nodiscard)
+#define LLVM_NODISCARD [[nodiscard]]
+#elif LLVM_HAS_CPP_ATTRIBUTE(clang::warn_unused_result)
+#define LLVM_NODISCARD [[clang::warn_unused_result]]
+// Clang in C++14 mode claims that it has the 'nodiscard' attribute, but also
+// warns in the pedantic mode that 'nodiscard' is a C++17 extension (PR33518).
+// Use the 'nodiscard' attribute in C++14 mode only with GCC.
+// TODO: remove this workaround when PR33518 is resolved.
+#elif defined(__GNUC__) && LLVM_HAS_CPP_ATTRIBUTE(nodiscard)
+#define LLVM_NODISCARD [[nodiscard]]
+#else
+#define LLVM_NODISCARD
+#endif
+
 // Indicate that a non-static, non-const C++ member function reinitializes
 // the entire object to a known state, independent of the previous state of
 // the object.