6.3: Fix KeyPath with 16-byte-aligned subscript traps on 32-bit targets

[MaxDesiatov]

Cherry-pick of #88725

• Explanation: Fixes a crash with key paths on 32-bit platforms reproducible for types that have 16-byte alignment. The intended bit layout of ComputedArgumentSize in KeyPath on 32-bit is:

 ┌───────┬───────────┐
 │ bits  │   field   │
 ├───────┼───────────┤
 │ 0–27  │ size      │
 ├───────┼───────────┤
 │ 28–29 │ padding   │
 ├───────┼───────────┤
 │ 30–31 │ alignment │
 └───────┴───────────┘

Currently, alignmentMask = 0x6000_0000, i.e. bits 29–30, not 30–31. It overlaps paddingMask (bits 28–29) at bit 29, meaning that alignment and padding unintentionally share a bit. With alignmentShift = 30, storing shift = 2 << 30 places 1 at bit 31, which the mask doesn't cover.

Correct value is 0xC000_0000 covers bits 30–31, which matches alignmentShift = 30 so both shift = 1 and shift = 2 round-trip, and it does not overlap with paddingMask = 0x3000_0000 (bits 28–29). It also mirrors the 64-bit layout (top bits of the word reserved for alignment, just 2 of them instead of 1).

• Scope: Limited to 32-bit platforms.
• Issues:
  rdar://175799967
  rdar://177213829
• Risk: Very low due to increased test coverage and sufficient incubation time on main and release/6.4.
• Testing: Previously crashing on 32-bit platforms sample code is now added to the test suite.
• Reviewers: @Azoy @kateinoigakukun

[MaxDesiatov]

@swift-ci test