`<fstream>`: Calling `tellg()` when reading an LF file in text mode causes `getline()` to loop forever

[StephanTLavavej]

Internal VSO-1841456 / AB#1841456 "[RWC][prod/fe][Regression] ITK failed to run test case itkCSVArray2DFileReaderTest" reported a runtime correctness regression caused by #3773. Here's a fully reduced test case:

D:\GitHub\STL\out\x64>type meow.cpp

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

void test(const ios_base::openmode mode) {
    {
        ofstream out{"delme.txt", mode};
        out << "Line A\n";
        out << "Line B\n";
        out << "Line C\n";
    }

    {
        ifstream in{"delme.txt"}; // Always reading in text mode!
        string line;
        for (int i = 0; i < 5 && getline(in, line); ++i) {
            cout << "Read: \"" << line << "\"; ";
            cout << "in.tellg(): " << in.tellg() << "\n";
        }
    }
}

int main() {
    cout << "Testing a text mode (CRLF) file.\n";
    test(ios_base::out);
    cout << "--------------------------------\n";
    cout << "Testing a binary mode (LF) file.\n";
    test(ios_base::binary);
}

Before

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /MTd /Od meow.cpp && meow
meow.cpp
Testing a text mode (CRLF) file.
Read: "Line A"; in.tellg(): 8
Read: "Line B"; in.tellg(): 16
Read: "Line C"; in.tellg(): 24
--------------------------------
Testing a binary mode (LF) file.
Read: "Line A"; in.tellg(): 5
Read: "Line B"; in.tellg(): 13
Read: "Line C"; in.tellg(): 21

After

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /MTd /Od meow.cpp && meow
meow.cpp
Testing a text mode (CRLF) file.
Read: "Line A"; in.tellg(): 8
Read: "Line B"; in.tellg(): 16
Read: "Line C"; in.tellg(): 24
--------------------------------
Testing a binary mode (LF) file.
Read: "Line A"; in.tellg(): 5
Read: "A"; in.tellg(): 5
Read: "A"; in.tellg(): 5
Read: "A"; in.tellg(): 5
Read: "A"; in.tellg(): 5