Disallow use of type variables for variables with static storage.

[dtarditi]

We cannot allow static variables to have types with free type variables. The storage for static variables in generic functions persists and could be used with different generic type instantiations. This creates a soundness issue.

Here is an example of how things can go wrong:

#include <stdbool.h>
#include <stdio.h>

_For_any(T) _Ptr<T> get_set(int  isSetter, _Ptr<T> val) {
	static _Ptr<T> capture;
	if (isSetter) {
		capture = val;
		return 0;
	}
	else
		return capture;
}

int main(int argc, _Array_ptr<_Nt_array_ptr<char>> argv : count(argc)) {
	short j = 0;
	short i = 1;
	get_set<short>(true, &i);
	_Ptr<double> p = get_set<double>(false, 0);
	*p = 3.141516; // corrupt memory
	printf(" j = %d", j);
}

When compiled by the Checked C clang compiler on Windows x64,, the store through p overwrites j. Here is the output.

D:\checkedc1\llvm\tools\clang\test\CheckedC>a.exe
 j = 9214