[IRGen][DebugInfo] Emit declarations for CFunctionPointer reps

[ellishg]

There is a verification pass to check that some DISubprograms have declarations.
swiftlang/llvm-project@1e608f9#diff-7f0021f2c6cd747015d1e98778f5d97944fd1ca5448e5750b3b1b637b0cf8323R1496-R1504

I suspect that 81953ef and later e0e7a08 aim to fix some of these cases. In the test case, I've managed to find a compiler-generated DISubprogram that fails this verification and I've found that adding a case for CFunctionPointer fixes this.

According to the comments, CFunctionPointer is nearly identical to CXXMethod, so I figured it was safe to add here.

swift/include/swift/AST/ExtInfo.h

Lines 258 to 261 in 795bfb9

	/// A C++ method that takes a "this" argument (not a static C++ method or
	/// constructor). Except for
	/// handling the "this" argument, has the same behavior as "CFunctionPointer".
	CXXMethod,

Resolves #74583

[ellishg]

Friendly ping @adrian-prantl @augusto2112 @dianqk

[ellishg]

Thanks for the accept! I believe I don't have access to test/merge. Can someone do that for me?

[NuriAmari]

@swift-ci Please test

[augusto2112]

@swift-ci smoke test

[augusto2112]

@ellishg thanks for fixing this!