build: allow falling back to just built compiler

When the native tools are sparse, and the toolchain is not being
cross-compiled, it is possible to fallback to the just built compiler
for running the tests.  Adjust the build rules to permit this.

(cherry picked from commit 86ddf43)

Author: compnerd

stdlib/cmake/modules/SwiftSource.cmake

@@ -580,7 +580,11 @@ function(_compile_swift_files
   endif()
 
   set(line_directive_tool "${SWIFT_SOURCE_DIR}/utils/line-directive")
-  set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc")
+  if(EXISTS "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc")
+    set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc")
+  elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL CMAKE_SYSTEM_NAME)
+    set(swift_compiler_tool "${SWIFT_RUNTIME_OUTPUT_INTDIR}/swiftc")
+  endif()
   set(swift_compiler_tool_dep)
   if(SWIFT_INCLUDE_TOOLS)
     # Depend on the binary itself, in addition to the symlink.

test/CMakeLists.txt

@@ -107,7 +107,11 @@ function(get_test_dependencies SDK result_var_name)
     list(APPEND deps ${deps_binaries})
   else()
     foreach(binary ${deps_binaries})
-      list(APPEND deps "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/${binary}${CMAKE_EXECUTABLE_SUFFIX}")
+      if(EXISTS "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/${binary}${CMAKE_EXECUTABLE_SUFFIX}")
+        list(APPEND deps "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/${binary}${CMAKE_EXECUTABLE_SUFFIX}")
+      elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL CMAKE_SYSTEM_NAME)
+        list(APPEND deps "${SWIFT_RUNTIME_OUTPUT_INTDIR}/${binary}${CMAKE_EXECUTABLE_SUFFIX}")
+      endif()
     endforeach()
   endif()