Convert the inner arrays of a nested constant array literal too

[ASDAlexander77]

Problem

#330 rebuilds a constant array literal element by element when the element type has to change, but skipped a literal whose elements are arrays. Such a literal keeps s32 inside its inner arrays:

const c = [[1, 2]]; let u: number[][] = c;   // u[0][1] read as garbage (-1.#QNAN)
let v: number[][] | string = [[1, 2]];       // did not compile at all

The union case failed module verification:

'ts.Cast' op type ['!ts.const_array<!ts.array<si32>, 1>'] can't be stored in ['!ts.union<!ts.string, !ts.array<!ts.array<!ts.number>>>']

Flat literals (#330), and nested literals of floats or strings — where no element type changes — were already fine.

Fix

• castConstArrayToArray turns a nested element back into a constant array and casts it in turn, so every level is rebuilt.
• canWideTypeWithoutDataLoss compares the element type inside nested array elements, through the new canWideConstArrayElementWithoutDataLoss, and only for a constant array source. This is what castToTaggedUnionType asks when it looks for the union member to cast to; without it no member matched and a cast from the literal straight to the union survived into verification. An array that is not a literal is still not converted element-wise, as before.

Tests

New 00const_array_nested_elements.ts (compile, JIT and corpus variants): a nested const variable (outer and inner length, inner element, and that elements really are number), a nested literal assigned to a number[][] | string union, and a three-level literal.

Full Windows debug suite (ctest -R "^test-|^unittest-"): 2765/2765 passed.

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