llvm: allow disabling split LTO unit for full LTO

[zhouguangyuan0718]

Summary

This PR adds a Full LTO bitcode writer binding for llgo and lets callers decide whether the generated summary index enables split LTO units.

Changes:

• add WriteFullLTOBitcodeToMemoryBuffer(m Module, enableSplitLTOUnit bool)
• build a full-LTO ModuleSummaryIndex and emit bitcode with the ThinLTO = 0 module flag
• only call Index.setEnableSplitLTOUnit() when enableSplitLTOUnit is true
• keep split-unit state in the summary index rather than exposing it as a module flag
• add a regression test for the full-LTO writer path

Motivation

llgo needs full-LTO bitcode for whole-program optimization, but Darwin's ld64.lld rejects mixed split-LTO-unit state with:

ld64.lld: error: inconsistent LTO Unit splitting (recompile with -fsplit-lto-unit)

Making split LTO unit emission explicit lets llgo use the same full-LTO writer on all platforms while disabling split units for Darwin links.

Test

go test