[clang] [CIR][ABI][NFC] Update ABI lowering design doc with implementation experience (PR #188617)

[via cfe-commits]

adams381 wrote:

Ping — this is a small NFC update to the ABI lowering design doc (ClangIRABILowering.md).  Three changes: Section 4.4 layered flow diagram, dialect-aware type mapping note, and ABIRewriteContext two-method interface description.  CI is green.  @andykaylor @bcardosolopes would either of you be willing to take a look?

https://github.com/llvm/llvm-project/pull/188617