Reinstate "SILCombine: Constant-fold MemoryLayout<T>.offset(of: \.literalKeyPath)"

[eeckstein]

This reinstates commit d7d829c of #32544 with a fix for C tail-allocated arrays.

Replace a call of the getter of AnyKeyPath._storedInlineOffset with a "constant" offset, in case of a keypath literal.
"Constant" offset means a series of struct_element_addr and tuple_element_addr instructions with a 0-pointer as base address.
These instructions can then be lowered to "real" constants in IRGen for concrete types, or to metatype offset lookups for generic or resilient types.

Replace:

  %kp = keypath ...
  %offset = apply %_storedInlineOffset_method(%kp)

with:

  %zero = integer_literal $Builtin.Word, 0
  %null_ptr = unchecked_trivial_bit_cast %zero to $Builtin.RawPointer
  %null_addr = pointer_to_address %null_ptr
  %projected_addr = struct_element_addr %null_addr
   ... // other address projections
  %offset_ptr = address_to_pointer %projected_addr
  %offset_builtin_int = unchecked_trivial_bit_cast %offset_ptr
  %offset_int = struct $Int (%offset_builtin_int)
  %offset = enum $Optional<Int>, #Optional.some!enumelt, %offset_int

rdar://problem/53309403

[eeckstein]

@swift-ci test