Use PROJECT_SOURCE_DIR for submodule compatibility

I used CMAKE_SOURCE_DIR to refer to the top-level source directory in
the ShardInstantiation.cmake file, but this can cause issues with
git submodules.  Instead, we should use PROJECT_SOURCE_DIR to ensure
compatibility when this project is used as a submodule in another
project.

Author: shumway

cmake/ShardInstantiation.cmake

@@ -81,7 +81,7 @@ function(generate_sharded_instantiations)
     foreach(SHARD_ID RANGE 0 ${LAST_SHARD_ID})
         set(NUM_SHARDS "${GEN_SHARDED_NUM_SHARDS}")
         set(SHARD_FUNCTION_PATH "${GEN_SHARDED_OUTPUT_DIR}/${INSTANCES}_shard_${SHARD_ID}.cpp")
-        set(SHARD_FUNCTION_TEMPLATE "${CMAKE_SOURCE_DIR}/cmake/instantiate_shard.in")
+        set(SHARD_FUNCTION_TEMPLATE "${PROJECT_SOURCE_DIR}/cmake/instantiate_shard.in")
         configure_file(
             "${SHARD_FUNCTION_TEMPLATE}"
             "${SHARD_FUNCTION_PATH}"
@@ -101,7 +101,7 @@ function(generate_sharded_instantiations)
 
     # Generate the caller function.
     set(CALLER_FUNCTION_PATH "${GEN_SHARDED_OUTPUT_DIR}/${INSTANCES}.cpp")
-    set(FUNCTION_TEMPLATE "${CMAKE_SOURCE_DIR}/cmake/call_shard.in")
+    set(FUNCTION_TEMPLATE "${PROJECT_SOURCE_DIR}/cmake/call_shard.in")
     configure_file(
         "${FUNCTION_TEMPLATE}"
         "${CALLER_FUNCTION_PATH}"