Feature Request: Warn when not using return value of pure method

**Product and Version Used**:
rosylnator, 2019

**Steps to Reproduce**:
string testString = "123 test";
testString.Replace("123", "");
output(testString);

**Actual Behavior**:
no warning that "Return value of pure method is not used"

**Expected Behavior**:
Warn me! "Return value of pure method is not used"

Begin with the C# reproduction in the issue and inspect the Roslynator analyzer entry points that handle method calls and diagnostics. Done means the shown unused pure-method return value produces the requested warning without incorrectly warning when the value is used; no specific files or tests are named.

Written by the indexing model from the issue text.