[Build] Pass the toolchain version through to Swift's compilation

Passes the value of the environment variable `TOOLCHAIN_VERSION` through
to Swift's `SWIFT_TOOLCHAIN_VERSION` for use in eg.
`-print-target-info` (and some day, `--version`).

Author: bnbarham

utils/build-script-impl

@@ -1566,6 +1566,13 @@ for host in "${ALL_HOSTS[@]}"; do
         )
     fi
 
+    if [[ "${TOOLCHAIN_VERSION}" ]] ; then
+        swift_cmake_options=(
+            "${swift_cmake_options[@]}"
+            -DSWIFT_TOOLCHAIN_VERSION:STRING="${TOOLCHAIN_VERSION}"
+        )
+    fi
+
     if [[ "${NATIVE_CLANG_TOOLS_PATH}" ]] ; then
         CLANG_BIN="${NATIVE_CLANG_TOOLS_PATH}"
         if [[ ! -f "${CLANG_BIN}/clang" ]] ; then

utils/build.ps1

@@ -1828,6 +1828,7 @@ function Get-CompilersDefines([Hashtable] $Platform, [switch] $Test) {
     Python3_INCLUDE_DIR = "$PythonRoot\include";
     Python3_LIBRARY = "$PythonRoot\libs\$PythonLibName.lib";
     Python3_ROOT_DIR = $PythonRoot;
+    SWIFT_TOOLCHAIN_VERSION = "${ToolchainIdentifier}";
     SWIFT_BUILD_SWIFT_SYNTAX = "YES";
     SWIFT_CLANG_LOCATION = (Get-PinnedToolchainToolsDir);
     SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY = "YES";