[embedded] Fix compiler asserts when building closures

[kubamracek]

See the added testcase, which previously triggered an assert. Two fixes:

• Improve detection of generic functions, just checking the SubstGenericSignature is flagging @substituted functions as generic even if they're specialized.
• Don't flag lazy emission of functions with external visibility because in embedded Swift, those can be gain public / externally-visible functions by deserializing them from imported modules, or by the CMO pass making local functions public. This is a workaround and not a real fix, it should probably be handled by some SIL pass that internalizes functions.

[kubamracek]

@swift-ci please test

[eeckstein]

lgtm