[CMake] Set -DLLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING=1 for runtime/overlays

Whenever we're building C++ code as part of the Swift runtime or overlays,
disable the ABI-breaking checks. We are only using header content from
LLVM's ADT library.

Fixes rdar://problem/48618250.

Author: DougGregor

cmake/modules/AddSwift.cmake

@@ -1209,6 +1209,15 @@ function(_add_swift_library_single target name)
     "${SWIFTLIB_SINGLE_FORCE_BUILD_OPTIMIZED_keyword}"
     RESULT_VAR_NAME c_compile_flags
     )
+
+  if(SWIFTLIB_IS_STDLIB)
+    # We don't ever want to link against the ABI-breakage checking symbols
+    # in the standard library, runtime, or overlays because they only rely
+    # on the header parts of LLVM's ADT.
+    list(APPEND c_compile_flags
+      "-DLLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING=1")
+  endif()
+
   if(SWIFTLIB_SINGLE_SDK STREQUAL WINDOWS)
     if(libkind STREQUAL SHARED)
       list(APPEND c_compile_flags -D_WINDLL)