Fix handling % chars in interpolated strings (#14440)

Interpolated strings require escaping '%' chars by doubling them,
however if an interpolated string literal had no interpolation
expressions, it would not drop extra '%' chars in the content.

Co-authored-by: Adam Boniecki <[email protected]>

Author: abonie

src/Compiler/Checking/CheckExpressions.fs

@@ -7165,13 +7165,14 @@ and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: Syn
         // Type check the expressions filling the holes
 
         if List.isEmpty synFillExprs then
-            let str = mkString g m printfFormatString
-
             if isString then
+                let sb = System.Text.StringBuilder(printfFormatString).Replace("%%", "%")
+                let str = mkString g m (sb.ToString())
                 TcPropagatingExprLeafThenConvert cenv overallTy g.string_ty env (* true *) m (fun () ->
                     str, tpenv
                 )
             else
+                let str = mkString g m printfFormatString
                 mkCallNewFormat g m printerTy printerArgTy printerResidueTy printerResultTy printerTupleTy str, tpenv
         else
             // Type check the expressions filling the holes

tests/FSharp.Compiler.ComponentTests/Language/InterpolatedStringsTests.fs

@@ -30,4 +30,10 @@ let b: System.IComparable = $"string"
 let c: System.IFormattable = $"string"
         """
         |> compile
-        |> shouldSucceed
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Percent sign characters in interpolated strings`` () =
+        Assert.Equal("%", $"%%")
+        Assert.Equal("42%", $"{42}%%")
+        Assert.Equal("% 42", $"%%%3d{42}")