[Clang][LoongArch] Implement patchable function entry

[xen0n]

Summary

Similar to D98610 for RISCV.

This is going to be required by the upcoming Linux/LoongArch
support for dynamic ftrace.

[SixWeining]

Does anyone know why patchable-function-entry is not documented in https://llvm.org/docs/LangRef.html#function-attributes?

llvm/lib/Target/LoongArch/LoongArchMCInstLower.cpp

150 ↗

(On Diff #489337)

Seems should return true? Consider when verifier is disabled.

How about handling PATCHABLE_FUNCTION_ENTER in LoongArchAsmPrinter.cpp like D140727?

[aaron.ballman]

The Clang bits LGTM, though you should add a release note about the new functionality so users know about it.

[xen0n]

llvm/lib/Target/LoongArch/LoongArchMCInstLower.cpp

150 ↗

(On Diff #489337)

I've checked other targets and indeed implementation in AsmPrinter is the more widespread approach (RISCV is the outlier here). Let me adjust in one or two days.

[xen0n]

Rebase and change to lower the op in LoongArchAsmPrinter so as to make people's lives easier when they come to rebase D141785 on top of this.

[xen0n]

Document the change in Clang release note.

[SixWeining]

LGTM. Thanks.