There are three major changes here:
1. The addition of "SILFunctionTypeRepresentation::CXXMethod".
2. C++ methods are imported with their members *last*. Then the arguments are switched when emitting the IR for an application of the function.
3. Clang decls are now marked as foreign witnesses.
These are all steps towards being able to have C++ protocol conformance.
This patch includes a large number of changes to make sure that:
1. When ExtInfo values are created, we store a ClangTypeInfo if applicable.
2. We reduce dependence on storing SIL representations in ASTExtInfo values.
3. Reduce places where we sloppily create ASTExtInfo values which should
store a Clang type but don't. In certain places, this is unavoidable;
see [NOTE: ExtInfo-Clang-type-invariant].
Ideally, we would check that the appropriate SILExtInfo does always store
a ClangTypeInfo. However, the presence of the HasClangFunctionTypes option
means that we would need to condition that assertion based on a dynamic check.
Plumbing the setting down to SILExtInfoBuilder's checkInvariants would be too
much work. So we weaken the check for now; we should strengthen it once we
"turn on" HasClangFunctionTypes and remove the dynamic feature switch.
In the future, we will remove the UseClangFunctionTypes language option, but we
temporarily need the scaffolding for equality checks to be consistent in all
places.
Since the two ExtInfos share a common ClangTypeInfo, and C++ doesn't let us
forward declare nested classes, we need to hoist out AnyFunctionType::ExtInfo
and SILFunctionType::ExtInfo to the top-level.
We also add some convenience APIs on (AST|SIL)ExtInfo for frequently used
withXYZ methods. Note that all non-default construction still goes through the
builder's build() method.
We do not add any checks for invariants here; those will be added later.
