Add analyzer: Warn on illegal format specifiers

[MaStr11]

Based on feedback from issue dotnet/roslyn#49471

Format specifiers are often compile-time constants that can be checked by an analyzer. This could be done in roslyn itself or as an SDK analyzer. For some specifiers a fix can be provided: dt.ToString("h") -> dt.ToString("%h").

var dt = new DateTime(2020, 12, 24, 6, 30, 45);
dt.ToString("h") // Fails at runtime
dt.ToString("h ")
dt.ToString("%h")
$"{dt:h}" // Fails
string.Format("{0:h}", dt) // Fails at runtime
string.Format("{0:/}", dt) // Fails at runtime
string.Format("{0::}", dt) // Fails at runtime
string.Format("{0:\\}", dt) // Fails at runtime
string.Format("{0:,}", dt) // Fails at runtime
string.Format("{0,d}", 2) // Fails at runtime
string.Format("{0", 3) // Fails at runtime

Sample on sharplab.io

The analyzer should detect format specifiers and check whether the specifier is valid. One possibility could be to simulate the behavior at runtime by passing default values:
Parsed code:

var dt = new DateTime(2020, 12, 24, 6, 30, 45);
string.Format("Hour of day {0:h}", dt);

In the analyzer the logic could be something like this:

...
InvocationExpressionSyntax stringFormatInvocation;
var firstArgument = stringFormatInvocation.ArgumentList[0];
var remainingArguments = stringFormatInvocation.ArgumentList[1..];
var formatString = semanticModel.GetConstantValue(firstArgument);
var args = remainingArguments.Select(a => 
    GetDefaultValueOfTypeIfCLRTypeOtherwiseNull(semanticModel.GetTypeInfo(a).Type)).ToArray();
try
{
  string.Format(formatString, args);
}
catch (Exception ex)
{
// Create a diagnostic.
}

If an custom format provider is used, the analyzer should do nothing.

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[buyaa-n]

Sounds like could be covered by #64009