Clone swift-syntax Alongside Swift

swift-syntax is going to own all of the infrastructure that
generates the syntax nodes, and the legacy libSyntax build
still depends on that infrastructure. To make this hang together,
we need to clone swift-syntax and point the compiler build into it.

This is being a little overly-aggressive by cloning swift-syntax
into strictly more places than is necessary. The idea being
that Swift itself will begin to require the Swift parser in more
parts of the build (cf Doug's changes to enable the "early swift syntax"
stage of the build).

Author: CodaFi

.ci/vs2022.yml

@@ -129,7 +129,11 @@ resources:
       name: unicode-org/icu
       ref: refs/heads/maint/maint-69
       type: github
-
+    - repository: apple/swift-syntax
+      endpoint: GitHub
+      name: apple/swift-syntax
+      ref: refs/heads/main
+      type: github
 
 stages:
   - stage: tools
@@ -148,6 +152,8 @@ stages:
             fetchDepth: 1
           - checkout: apple/swift-corelibs-libdispatch
             fetchDepth: 1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - script: |
               SET vswhere="%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
               FOR /f "usebackq delims=" %%i IN (`%vswhere% -latest -property installationPath`) DO (
@@ -189,6 +195,7 @@ stages:
                 -D SWIFT_BUILD_STATIC_STDLIB=NO
                 -D SWIFT_BUILD_DYNAMIC_SDK_OVERLAY=NO
                 -D SWIFT_BUILD_STATIC_SDK_OVERLAY=NO
+                -D SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE=$(Build.SourcesDirectory)/swift-syntax
           - task: CMake@1
             inputs:
               cmakeArgs:
@@ -258,6 +265,8 @@ stages:
             fetchDepth: 1
           - checkout: apple/swift-experimental-string-processing
             fetchDepth: 1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - powershell: |
               if ("$(arch)" -eq "amd64") {
                 $ArchComponent = ".x86.x64"
@@ -364,6 +373,7 @@ stages:
                 -D SWIFT_PARALLEL_LINK_JOBS=2
                 -D SWIFT_BUILD_DYNAMIC_STDLIB=NO
                 -D SWIFT_BUILD_DYNAMIC_SDK_OVERLAY=NO
+                -D SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE=$(Build.SourcesDirectory)/swift-syntax
                 $(EXTRA_CMAKE_ARGS)
           - task: CMake@1
             inputs:
@@ -808,6 +818,8 @@ stages:
             fetchDepth: 1
           - checkout: apple/swift-experimental-string-processing
             fetchDepth: 1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - powershell: |
               if ("$(arch)" -eq "amd64" -Or "$(arch)" -eq "x86") {
                 $ArchComponent = ".x86.x64"
@@ -887,6 +899,7 @@ stages:
                 -D SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING=YES
                 -D SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING=YES
                 -D EXPERIMENTAL_STRING_PROCESSING_SOURCE_DIR=$(Build.SourcesDirectory)/swift-experimental-string-processing
+                -D SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE=$(Build.SourcesDirectory)/swift-syntax
           - task: CMake@1
             inputs:
               cmakeArgs:
@@ -1068,6 +1081,8 @@ stages:
             fetchDepth: 1
           - checkout: jpsim/Yams
             fetchDepth: 1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - script: |
               SET vswhere="%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
               FOR /f "usebackq delims=" %%i IN (`%vswhere% -latest -property installationPath`) DO (
@@ -1470,6 +1485,8 @@ stages:
             fetchDepth: 1
           - checkout: apple/swift
             fetchDepth:  1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - task: MSBuild@1
             inputs:
               solution: $(Build.SourcesDirectory)/swift-installer-scripts/platforms/Windows/toolchain.wixproj
@@ -1539,6 +1556,8 @@ stages:
             fetchDepth: 1
           - checkout: apple/swift
             fetchDepth: 1
+          - checkout: apple/swift-syntax
+            fetchDepth: 1
           - task: MSBuild@1
             inputs:
               solution: $(Build.SourcesDirectory)/swift-installer-scripts/platforms/Windows/CustomActions/SwiftInstaller/SwiftInstaller.vcxproj