[impellerc] Crash on Windows when paths contain non-ASCII characters #190233

The Impeller shader compiler (impellerc) crashes (exit code 3) on Windows when include paths contain non-ASCII (Unicode) characters.
This happens because the C++ command line parser in engine/src/flutter/impeller/compiler/switches.cc converts path strings using narrow ANSI strings instead of UTF-8, which then fails inside fml::Utf8ToWideString.

Additionally, flutter_tools hides impellerc's stdout and stderr during ShaderCompilerException, making this hard to debug.
We need to:

1. Fix the path conversion in switches.cc to use Utf8FromPath.
2. Improve ShaderCompilerException diagnostics to capture and print stdout/stderr.
3. Add helpful hints in the tool for aborted compilations and Unicode path warnings.

Note: This issue was identified as a top-10 crasher in the Flutter stable release version 3.44.8.