[Backtracing] Fix a Windows build issue.

When backtracing is disabled, don't try to refer to
`_swift_backtraceSettings`.

rdar://117470489

Author: al45tair

stdlib/public/runtime/CMakeLists.txt

@@ -141,6 +141,7 @@ add_swift_target_library(swiftRuntime OBJECT_LIBRARY
   ${swift_runtime_threading_sources}
   C_COMPILE_FLAGS
     ${swift_runtime_library_compile_flags}
+    ${swift_enable_backtracing}
   LINK_FLAGS ${swift_runtime_linker_flags}
   SWIFT_COMPILE_FLAGS ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
   INSTALL_IN_COMPONENT never_install)

stdlib/public/runtime/Errors.cpp

@@ -354,7 +354,7 @@ void swift::swift_reportError(uint32_t flags,
                               const char *message) {
 #if defined(__APPLE__) && NDEBUG
   flags &= ~FatalErrorFlags::ReportBacktrace;
-#else
+#elif SWIFT_ENABLE_BACKTRACING
   // Disable fatalError backtraces if the backtracer is enabled
   if (runtime::backtrace::_swift_backtrace_isEnabled()) {
     flags &= ~FatalErrorFlags::ReportBacktrace;