[ClangImporter] NFC: extract Swift decl synthesis logic into a separate file

`ImportDecl.cpp` contained 10k+ lines of code, which caused slowdowns in incremental compilation and while editing the code in the IDE.

This change extracts a chunk of largely self-contained decl synthesis logic into a separate file.

Author: egorzhdan

lib/ClangImporter/CMakeLists.txt

@@ -13,6 +13,7 @@ add_swift_host_library(swiftClangImporter STATIC
   ClangImporterRequests.cpp
   ClangModuleDependencyScanner.cpp
   ClangSourceBufferImporter.cpp
+  SwiftDeclSynthesizer.cpp
   DWARFImporter.cpp
   ImportDecl.cpp
   ImportEnumInfo.cpp