[AutoDiff] [SR-14240] Align name mangling for Clang-imported declarations

[BradLarson]

When importing Clang declarations, DifferentiationMangler handles their non-mangled names differently than other mangled functions. ASTMangler currently does not, and as a consequence createEmptyVJP() and createEmptyJVP() add empty JVPs and VJPs with different mangled names for IR and TBD.

When you try to add a custom VJP to one of these imported functions, like in the following:

import Darwin

@inlinable
@derivative(of: pow)
func powVJP(
  _ base: Double, _ exponent: Double
) -> (value: Double, pullback: (Double) -> (Double, Double)) {
  fatalError()
}

You hit an error of error: symbol 'powTJfSSpSr' (powTJfSSpSr) is in generated IR file, but not in TBD file due to the matching empty JVP that is created.

This PR has ASTMangler::beginManglingWithAutoDiffOriginalFunction() detect imported Clang functions and mangles them in the same way as DifferentiationMangler. That then prevents the difference in symbols that triggers the above error.

Resolves SR-14240.

[BradLarson]

@swift-ci Please test.

[dan-zheng]

The fix description makes sense to me, thank you!

I haven't touched mangling in a while, perhaps @rxwei could also help review?