This is conditional on UseAsyncLowering and in the future should also be
conditional on `clangTargetInfo.isSwiftAsyncCCSupported()` once that
support is merged.
Update tests to work either with swiftcc or swifttailcc.
Previously, the "bare" linkage of a link entity was used to determine
whether to put an async function pointer into the tbd. That did not
match the mechanism by which the linkage was determined in IRGen.
There, the linkage is the "_effective_" linkage (i.e. the value returned
from SILFunction::getEffectiveSymbolLinkage).
Here, whether to put the async function pointer corresponding to a class
method is determined on the basis of that effective linkage.
rdar://problem/73203508