template specialization in header is mishandled

[shivanshu3]

Given the following input, IWYU produces an incorrect output:

main.cpp:

struct Foo
{
	static constexpr int num = 10;
};

#include "NumGetter.h"

#include "IntegerWrapperFoo.h"

int dummy()
{
	NumGetter<Foo> x;
	return x.GetNum();
}

NumGetter.h:

template<typename T>
struct IntegerWrapper
{};

template<typename T>
struct NumGetter
{
	int GetNum()
	{
		return IntegerWrapper<T>::num;
	}
};

IntegerWrapperFoo.h:

template<>
struct IntegerWrapper<Foo>
{
	static constexpr int num = Foo::num;
};

IWYU output:

>include-what-you-use.exe main.cpp

main.cpp should add these lines:

main.cpp should remove these lines:
- #include "IntegerWrapperFoo.h"  // lines 8-8

The full include-list for main.cpp:
#include "NumGetter.h"  // for NumGetter
---

I am on the master branch of IWYU at commit b4838853e5c906854a616d9e975b53c164950174 from May 24 2020.

[shivanshu3]

@kimgr I tried fixing this, and I've submitted a PR (#803)
Let me know what you think about this change. I applied this change on our internal fork of IWYU and it seems to work well and fixes the issue I was seeing originally.

If you think I'm on the right track, I will add a test and also look at some test failures. 22 tests were failing before my change, and 28 are failing after this change.