Test limitation: %D3DReflect only loads from prior stage stdout, preventing IR-based test

[tex3d]

Description
In FileCheckerTest.cpp, %D3DReflect requires the use of %s argument, but does not load contents from the file. Instead, it loads from prior stage's stdout (Prior->StdOut). If you start from IR, there's no way to get your IR into the StdOut for the next stage, making it difficult or impossible to feed it to %D3DReflect. We should fix this handling so it accepts from prior stage's stdout (Prior->StdOut) only when %s is not supplied. Then when %s is supplied, use the file as the input instead.

A temporary workaround for this was added in #6207 and commented on here and here. This workaround will load the file if the output from the prior stage is empty, since otherwise empty input causes a crash anyway.

Steps to Reproduce

; RUN: %dxilver 1.6 | %D3DReflect %s | FileCheck %s -check-prefixes=RDAT,RDAT16
; followed by an IR based test

Fixing this will require an update of a bunch of tests that all use the form %D3DReflect %s to remove the %s.

Actual Behavior
Crash, or workaround in #6207 will make this work, but it's not selecting input based on whether %s is present like it should.

Environment

• DXC version: any recent
• Host Operating System: any