[cxx-interop] Fix AST serialization error

Do not attempt to serialize C++ builtin decls. This caused an assertion failure in `clang::ASTWriter::getDeclID` when building SwiftCompilerSources:

```
Assertion failed: (DeclIDs.find(D) != DeclIDs.end() && "Declaration not emitted!"), function getDeclID, file ASTWriter.cpp, line 5272.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the project and the crash backtrace.
Stack dump:
0.	<eof> parser at end of file
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x00000001123e21c7 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 39
1  swift-frontend           0x00000001123e10e8 llvm::sys::RunSignalHandlers() + 248
2  swift-frontend           0x00000001123e2810 SignalHandler(int) + 288
3  libsystem_platform.dylib 0x00007fff204a0d7d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000000008 _sigtramp + 18446603339974439592
5  libsystem_c.dylib        0x00007fff203b0406 abort + 125
6  libsystem_c.dylib        0x00007fff203af7d8 err + 0
7  swift-frontend           0x000000011345b983 clang::ASTWriter::getDeclID(clang::Decl const*) (.cold.1) + 35
8  swift-frontend           0x0000000110a584b2 clang::ASTWriter::getDeclID(clang::Decl const*) + 322
9  swift-frontend           0x000000010e36649c llvm::OnDiskChainedHashTableGenerator<(anonymous namespace)::BaseNameToEntitiesTableWriterInfo>::Emit(llvm::raw_ostream&, (anonymous namespace)::BaseNameToEntitiesTableWriterInfo&) + 1244
10 swift-frontend           0x000000010e364927 swift::SwiftLookupTableWriter::writeExtensionContents(clang::Sema&, llvm::BitstreamWriter&) + 1351
11 swift-frontend           0x0000000110a59b62 clang::ASTWriter::WriteModuleFileExtension(clang::Sema&, clang::ModuleFileExtensionWriter&) + 946

```

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -2629,6 +2629,10 @@ bool importer::shouldSuppressDeclImport(const clang::Decl *decl) {
     return false;
   }
 
+  if (isa<clang::BuiltinTemplateDecl>(decl)) {
+    return true;
+  }
+
   return false;
 }