[build-presets] Test Swift Syntax during macOS PR smoke testing

There have been occasions in the past where changes to the
gyb_syntax_support broke the SwiftSyntax build that weren't caught in CI
because smoke test didn't build/test the downstream Swift Syntax project.

Author: ahoppen

utils/build-presets.ini

@@ -588,13 +588,15 @@ build-ninja
 libcxx
 llbuild
 swiftpm
+swiftsyntax
 swift-driver
 indexstore-db
 sourcekit-lsp
 install-llvm
 install-swift
 install-llbuild
 install-swiftpm
+install-swiftsyntax
 install-swift-driver
 install-libcxx