x/tools/gopls: unusedparams analyzer reports false positive due to build tags

[adonovan]

xtools$ go run ./gopls/internal/analysis/unusedparams/cmd/main.go -c 3 ./gopls/internal/test/integration/fake
/Users/adonovan/w/xtools/gopls/internal/test/integration/fake/workdir.go:75:38: unused parameter: err
72	
73	// isWindowsErrLockViolation reports whether err is ERROR_LOCK_VIOLATION
74	// on Windows.
75	var isWindowsErrLockViolation = func(err error) bool { return false }
76	

The analysis believes that this function is assigned to a variable that is only ever used in call position--because it is never used at all. In fact it is used in non-call position, for an assignment to the variable, though only in a file tagged for GOOS=windows, which of course the analysis cannot see. Consequently it falsely reports that the parameter can be safely removed.

One possibility would be to observe that pass.IgnoredGoFiles contains one or more files, in particular files with build tags, and to conservatively assume that these extra files contain assignments to all variables such as isWindowsErrLockViolation.

This is really a representative for a whole class of built-tag-related analyzer problems.

[findleyr]

Moving this to the backlog, since this is a large category of problem that requires careful thought and planning.

[adonovan]

Closely related to #71686.