The function declaration `f` in the following example compiles on both MSVC 19.16 and GCC 8.2 [1]:

test.h:
```
typedef void CB(int i);

void f(CB x);
```

But jextract rejects it with the following error:

```
> jextract -J-D"jextract.debug=true" test.h
Unsupported type kind: FunctionProto
java.lang.IllegalArgumentException: Unsupported type kind: FunctionProto
        at jdk.jextract/com.sun.tools.jextract.tree.LayoutUtils.getLayout(LayoutUtils.java:181)
        at jdk.jextract/com.sun.tools.jextract.tree.LayoutUtils.getLayout(LayoutUtils.java:163)
        at jdk.jextract/com.sun.tools.jextract.tree.LayoutUtils.parseFunctionInternal(LayoutUtils.java:120)
        at jdk.jextract/com.sun.tools.jextract.tree.LayoutUtils.getFunction(LayoutUtils.java:109)
        at jdk.jextract/com.sun.tools.jextract.Utils.getFunction(Utils.java:129)
        at jdk.jextract/com.sun.tools.jextract.TypeDictionary.getInternal(TypeDictionary.java:145)
        at jdk.jextract/com.sun.tools.jextract.TypeDictionary.enterIfAbsent(TypeDictionary.java:201)
        at jdk.jextract/com.sun.tools.jextract.TypeEnter.visitFunction(TypeEnter.java:92)
        at jdk.jextract/com.sun.tools.jextract.TypeEnter.visitFunction(TypeEnter.java:33)
        at jdk.jextract/com.sun.tools.jextract.tree.FunctionTree.accept(FunctionTree.java:48)
        at jdk.jextract/com.sun.tools.jextract.JextractTool.lambda$generateHeader$4(JextractTool.java:75)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
        at jdk.jextract/com.sun.tools.jextract.JextractTool.generateHeader(JextractTool.java:75)
        at jdk.jextract/com.sun.tools.jextract.JextractTool.lambda$processHeaders$3(JextractTool.java:69)
        at java.base/java.util.HashMap.forEach(HashMap.java:1333)
        at jdk.jextract/com.sun.tools.jextract.JextractTool.processHeaders(JextractTool.java:69)
        at jdk.jextract/com.sun.tools.jextract.Main.run(Main.java:264)
        at jdk.jextract/com.sun.tools.jextract.Main.main(Main.java:334)
```

[1]: https://godbolt.org/z/PJIofg