Runtimes: add -nostdlibimport to the runtime build

If the compiler being used to build the runtime is from a complete
toolchain, we may inadvertently pick up runtime from the previous build
rather than the just built Swift Core library. Pass `-nostdlibimport` to
ensure that we do not search the default search paths when building and
use the locally built swiftCore.

Author: compnerd

Runtimes/Core/CMakeLists.txt

@@ -172,6 +172,7 @@ add_compile_options(
 add_link_options($<$<PLATFORM_ID:Windows>:LINKER:/WX>)
 
 add_compile_options(
+  "$<$<COMPILE_LANGUAGE:Swift>:-nostdlibimport>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-library-level api>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-runtime-compatibility-version none>"
   "$<$<COMPILE_LANGUAGE:Swift>:-disable-autolinking-runtime-compatibility-dynamic-replacements>"