Avoid DirectorySeparatorChar.ToString()

These codepaths all allocate a string unnecessarily. `EndsWith(char)` is not available in .NET Framework but they could meantime be replaced with something like FileUtilities.EnsureTrailingSlash. Note that some check for AltDirectorySeparatorChar and some do not though.

```
C:\git\msbuild\src\Build\Construction\Solution\SolutionProjectGenerator.cs:
2198: if (!directoryName.EndsWith(Path.DirectorySeparatorChar.ToString(), StringComparison.Ordinal))

C:\git\msbuild\src\Shared\FrameworkLocationHelper.cs:
487 FallbackDotNetFrameworkSdkInstallPath.EndsWith(
488: Path.DirectorySeparatorChar.ToString(),
489 StringComparison.Ordinal);
...
960: var endedWithASlash = path.EndsWith(Path.DirectorySeparatorChar.ToString(), StringComparison.Ordinal)
961 || path.EndsWith(
962 Path.AltDirectorySeparatorChar.ToString(),

C:\git\msbuild\src\Utilities\ToolLocationHelper.cs:
2114 !dotNetFrameworkReferenceAssemblies[i].EndsWith(
2115: Path.DirectorySeparatorChar.ToString(),
2116 StringComparison.Ordinal))
```

Most likely this is more tidiness than any measurable perf impact.