System.Private.Xml: Use Encoding.Preamble (#23323)

* System.Private.Xml: Use Encoding.Preamble

Use the new `Encoding.Preamble` property to avoid unnecessary `byte[]`
allocations for encodings that return cached instances from `Preamble`
(e.g. all of the built-in encodings that have preambles).

* Avoid TypeLoadException in async code

Factor out the "eat preamble" code into its own method (that can be
reused by both the sync and async code) to prevent the
`ReadOnlySpan<byte>` preamble from being captured in a field of the
async state machine. This avoids the System.TypeLoadException : A value
type containing a by-ref instance field, such as Span<T>, cannot be used
as the type for a class instance field.

0 files changed, 0 insertions, 0 deletions