Split MLIRGen.cpp: extract MLIRGenImpl into private header, move cast family to MLIRGenCast.cpp (review doc 1)

[ASDAlexander77]

First mechanical slice of the §1 TU split from docs/MLIRGen-refactoring-review.md.

What

• MLIRGenImpl.h (new, private to lib/TypeScript): the whole MLIRGenImpl class moved out of MLIRGen.cpp's anonymous namespace into typescript::mlirgen. Bodies remain inline for now; each following slice replaces one family's inline bodies with declarations and moves the definitions into a new TU.
• MLIRGenCast.cpp (new): first satellite TU — the contiguous cast family (selectFieldsValues … createBoundMethodFromExtensionMethod, 29 methods, ~1,600 lines) defined out-of-line. Default arguments stay on the declarations only.
• MLIRGen.cpp shrinks to 101 lines: compileOptionsPtr static + getCompileOptions()/new setCompileOptions(), SourceMgrDiagnosticHandlerEx methods, and the two public entry points.

Enabling fixes (single-TU assumptions the split exposed)

• GenContext/PassResult/ValueOrLogicalResult were defined in an anonymous namespace inside a header (MLIRGenContext.h), so every TU got its own distinct types — fine with one TU, an ODR/link break with two. They now live in namespace typescript (the header already had using namespace ::typescript; above, so all unqualified uses keep working).
• ts::print(Node) in ts-new-parser/dump.h was a non-inline function defined in a header → multiply defined; now inline.
• The constructor no longer touches the TU-static compileOptionsPtr directly (inline code referencing a TU-static would be an ODR trap); it calls setCompileOptions() defined in MLIRGen.cpp.

Verification

• Debug build clean (the earlier C5046 internal-linkage warning is gone).
• MLIRGenTests 12/12.
• Full ctest suite: JIT 339/339, AOT 342/342.

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