runtime: fix build of the runtime tests for Windows

Ensure that the local definition of the stdlib functions are marked for
export.  We would previously mark the runtime functions as dllimport on
Windows.

Author: compnerd

unittests/runtime/CMakeLists.txt

@@ -47,6 +47,13 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
     $<TARGET_OBJECTS:swiftRuntime${SWIFT_PRIMARY_VARIANT_SUFFIX}>
     )
 
+  # The local stdlib implementation provides definitions of the swiftCore
+  # interfaes to avoid pulling in swiftCore itself.  Build the SwiftRuntimeTests
+  # with swiftCore_EXPORTS to permit exporting the stdlib interfaces.
+  target_compile_definitions(SwiftRuntimeTests
+                             PRIVATE
+                               swiftCore_EXPORTS)
+
   # FIXME: cross-compile for all variants.
   target_link_libraries(SwiftRuntimeTests
     swiftCore${SWIFT_PRIMARY_VARIANT_SUFFIX}