[x64][win] Windows x64 unwind v3: Use tail-relative epilog offsets and add size-based splitting (#203108)

Win64 Unwind v3 encodes each epilog's EpilogOffset as a signed 16-bit
field. The encoder previously measured the first epilog offset from the
fragment start, which overflowed for large functions and produced a
cryptic "<unknown>:0: value too large for field" error (and, on the
early .seh_handlerdata path, an assertion failure).

Two changes:

- MCWin64EH.cpp: Always emit epilog offsets tail-relative. The first
epilog descriptor is measured from the fragment end and subsequent ones
as deltas from the previous epilog, so descriptors are emitted in
descending address order (all non-positive, per spec). A new lazy
MCUnwindV3EpilogOffsetTargetExpr resolves the fragment-end-relative
value at layout time (it may not have a symbol yet when emitted via
.seh_handlerdata) and reports a clean, function-named diagnostic on
genuine overflow.

- X86WinEHUnwindV3.cpp: Add Unwind-v2-style size-based sub-fragment
splitting. In addition to the existing >7-epilog split, the pass now
splits a funclet into chained fragments when an epilog is too far from
its fragment tail (using the V2 approximate-instruction-count heuristic,
tunable via -x86-wineh-unwindv3-epilog-distance-threshold). Splits use
SEH_SplitChainedAtEndOfBlock so the split lands after the epilog,
keeping every epilog close to its tail; a long trailing region becomes
its own epilog-free chained fragment.

Updates seh-unwindv3-large.s expectations to the new tail-relative
values and adds win64-eh-unwindv3-split-large.ll covering size-based
splitting.