Remove std::function churn and wasted arena interning in MLIRGen

[ASDAlexander77]

Summary

Implements item A4 (allocation hot spots) from docs/MLIRGen-refactoring-review.md:

• Utility-type dispatch tables. getEmbeddedTypeWithParamBuiltins / ...NoBuiltins / ...ManyParamsBuiltins used llvm::StringSwitch<std::function<...>>, which constructs every case's closure eagerly on each call (~17 std::functions per getType of a builtin utility type). Rewritten as StringSwitch<enum> + a plain switch — allocation-free, and the shared "evaluate first type argument" prologue is now hoisted once. Behavior notes: cases that previously returned a possibly-null type without checking (Readonly/Partial/Required/ThisType) are unchanged in effect, since returning a null mlir::Type equals returning mlir::Type(); the NoBuiltins variant deliberately keeps no null check, matching the original.
• std::bind elimination (18 sites → 0). The four verbatim 5-placeholder cast binds became one explicit lambda; the two anyOrUndefined/optionalValueOrUndefined ternaries became member-function-pointer selection (identical signatures) invoked via this->*; the constructor's four MLIRTypeHelper callbacks and the getGlobalsFullNamespaceName bind became lambdas.
• Arena interning. getFullNamespaceName(name) permanently interned its result into the never-freed BumpPtrAllocator on every call. New concatFullNamespaceName() builds the name without interning and is used where the result is transient — getNameOfFunction (which immediately called .str()), the two enum FlatSymbolRefAttr sites (MLIR interns into the context anyway), and the dynamic-import symbol name. Registration paths keep interning.

Test plan

• tslang rebuilt cleanly (vs2026 debug)
• New smoke test covering all three rewritten dispatch tables (Uppercase, Capitalize, Pick, Record, Readonly, ReturnType, Exclude), spread-into-fixed-params (member-pointer valueFactory path), Array.push (cast-lambda path), enums and namespaced functions (concatFullNamespaceName paths) — JIT output 5 1 / ABC Hello 1 x 3 true c / 6 / 42, all as expected
• Prior smoke tests for Refactor theModule handling in MLIRGenImpl #201/Add GenContext field initializers and SourceFileScope RAII guard #202 paths still produce 42 15 42 3 1 and 5 21 7 x
• MLIRGenTests.exe: 12/12 pass

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