Don't include empty-string arguments in compile_commands.json

compdb.go was incorrectly splitting compiler arguments. This could
cause empty strings to be included in compile_commands.json,
potentially confusing tools.

Bug: 117124308
Test: m checkbuild
Test: m SOONG_GEN_COMPDB=1 \
        SOONG_GEN_COMPDB_DEBUG=1 \
        SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
      Examine $ANDROID_BUILD_TOP/compile_commands.json
Change-Id: I375baf255c50a1329cd644ac584d200aba9daa69

cc/compdb.go

@@ -181,7 +181,7 @@ func generateCompdbProject(compiledModule CompiledInterface, ctx android.Singlet
 func evalAndSplitVariable(ctx android.SingletonContext, str string) ([]string, error) {
 	evaluated, err := ctx.Eval(pctx, str)
 	if err == nil {
-		return strings.Split(evaluated, " "), nil
+		return strings.Fields(evaluated), nil
 	}
 	return []string{""}, err
 }