Initialize XmlReaders using StreamReaders

[benvillalobos]

Fixes #6847

Context

When XmlReader.Create is called and a file path is passed in, it's converted to a URI under the hood. This can mangle multi-byte characters. The solution (in the link above) is to initialize the XmlReader using a stream to the file instead of the path alone.

Changes Made

Call XmlReader.Create with a stream that will detect encodings.

Testing

Tested using repro in #6847

Notes

[benvillalobos]

Digging into the error a bit more, it looks like initializing the XmlReader with a FileScheme Uri causes funky behavior when transforming an Xslt doc that calls the Document function. The Document function looks up data in another document.

From what I can tell, loading the document from a stream is causing issues when calling functions from the loaded xslt. When Document is called from a.xml and looks for b.xml, it now tries to find b.xml in the output bin directory from the current unit test project, instead of temp where the files were generated.

---

A quick chat with Rainer showed that there's an overload to XmlReader.Create that takes a base path, and passing the original filepath ensures the path stays relative, solving the Document issue and the mangled character issue 🎉

[benvillalobos]

To-Do: Test an xslt doc calling the Document method for a file in the same unicode-style path that causes the error.

[benvillalobos]

To-Do: Test an xslt doc calling the Document method for a file in the same unicode-style path that causes the error.

It fails

"C:\src\temp\9-16\11_25\new\???????????????'`?-?        ????U1[?????]U2[????]U3[?????]\proj1\proj1.csproj" (default target) (1) ->
(Foo target) ->
  C:\src\temp\9-16\11_25\new\???????????????'`?-?       ????U1[?????]U2[????]U3[?????]\proj1\proj1.csproj(11,5): error MSB3703: Unable to execute transformation. An error occurred while loading document 'b.xml'. See InnerException for a c
omplete description of the error.

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:02.53

We'd need to catch this exception and output a better string here.

[benvillalobos]

imo we merge this and file an issue in the runtime tracking this

[benvillalobos]

Issue filed: dotnet/runtime#59353