Avoid const_cast in methods on PEDecoder

[Copilot]

Description

PEDecoder uses const_cast<PEDecoder *>(this)-> in 8 places across const methods to lazily cache validation flags and header pointers. This is the textbook use case for mutable.

Mark the four cache fields as mutable and remove all const_cast usages:

• src/coreclr/inc/pedecoder.h — Add mutable to m_flags, m_pNTHeaders, m_pCorHeader, m_pReadyToRunHeader
• src/coreclr/utilcode/pedecoder.cpp — Remove 7 const_cast sites (HasNTHeaders, CheckNTHeaders, CheckCorHeader, CheckILOnly ×2, FindReadyToRunHeader ×2)
• src/coreclr/inc/pedecoder.inl — Remove 1 const_cast site (GetCorHeader)

No layout or behavioral change — mutable only affects const-correctness at compile time.

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[adamperlin]

Pending CI, this LGTM. As copilot mentions, this is a compile time const-checking change so it shouldn't be behavioral.

[Copilot]

Pull request overview

This PR improves const-correctness within PEDecoder by replacing const_cast<PEDecoder*>(this) used for lazy caching inside const methods with mutable cache fields, keeping runtime behavior unchanged while making the intent explicit at the type level.

Changes:

• Mark PEDecoder cache members (m_flags, m_pNTHeaders, m_pCorHeader, m_pReadyToRunHeader) as mutable.
• Remove const_cast<PEDecoder*>(this)->... writes in pedecoder.cpp const methods used for one-time validation/caching.
• Remove the const_cast-based lazy initialization in PEDecoder::GetCorHeader().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/utilcode/pedecoder.cpp	Removes const_cast from const methods while preserving lazy caching of header pointers and validation flags.
src/coreclr/inc/pedecoder.inl	Updates GetCorHeader() to lazily cache via mutable member instead of const_cast.
src/coreclr/inc/pedecoder.h	Marks cache fields as mutable to support lazy initialization in const methods without casting away constness.

[AaronRobinsonMSFT]

I like this idea a lot, it is a good start. The real fix though is to remove the const from methods that are mutating. I realize that would be a much bigger change though.

[adamperlin]

/ba-g infra failures