fix issues related to export & function (#2002)

- `unused` function names
- confusion with function call syntax

fixes #2001

test/compress/issue-2001.js

@@ -0,0 +1,63 @@
+export_func_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        export function f(){};
+    }
+    expect_exact: "export function f(){};"
+}
+
+export_func_2: {
+    options = {
+        side_effects: false,
+        unused: true,
+    }
+    input: {
+        export function f(){}(1);
+    }
+    expect_exact: "export function f(){};1;"
+}
+
+export_func_3: {
+    options = {
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        export function f(){}(1);
+    }
+    expect_exact: "export function f(){};"
+}
+
+export_default_func_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        export default function f(){};
+    }
+    expect_exact: "export default function(){};"
+}
+
+export_default_func_2: {
+    options = {
+        side_effects: false,
+        unused: true,
+    }
+    input: {
+        export default function f(){}(1);
+    }
+    expect_exact: "export default function(){};1;"
+}
+
+export_default_func_3: {
+    options = {
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        export default function f(){}(1);
+    }
+    expect_exact: "export default function(){};"
+}