[swiftinterface] Handle target variants the same as targets (#77156)

Based on preliminary work from @rmaz.

The compilation arguments for a swiftinterface file are preprocessed to
modify the `-target` argument to match the preferred target (which comes
from the command line) in cases in which the sub-architecture differs,
but it is compatible (for example using `arm64e` when `arm64` is being
compiled), but this was not done for the target variant, which ended up
with mismatches on the sub-architecture used by the target and target
variant, which fails an assert in assert toolchains.

include/swift/Serialization/SerializedModuleLoader.h

@@ -571,7 +571,8 @@ public:
 bool extractCompilerFlagsFromInterface(
     StringRef interfacePath, StringRef buffer, llvm::StringSaver &ArgSaver,
     SmallVectorImpl<const char *> &SubArgs,
-    std::optional<llvm::Triple> PreferredTarget = std::nullopt);
+    std::optional<llvm::Triple> PreferredTarget = std::nullopt,
+    std::optional<llvm::Triple> PreferredTargetVariant = std::nullopt);
 
 /// Extract the user module version number from an interface file.
 llvm::VersionTuple extractUserModuleVersionFromInterface(StringRef moduleInterfacePath);