Enable the static Linux SDK build

Helps keep the codebase free of unintentional Glibc-isms. There are also some interesting testing use cases enabled by building Swift Build using the static SDK.

Author: jakepetroules

.github/workflows/pull_request.yml

@@ -19,6 +19,8 @@ jobs:
       linux_swift_versions: '["nightly-main", "nightly-6.2"]'
       windows_swift_versions: '["nightly-main"]'
       windows_build_command: 'swift test --no-parallel'
+      enable_linux_static_sdk_build: true
+      linux_static_sdk_build_command: SWIFTBUILD_STATIC_LINK=1 LLBUILD_STATIC_LINK=1 swift build
   cmake-smoke-test:
     name: cmake-smoke-test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main

Package.swift

@@ -20,6 +20,7 @@ let appleOS = true
 let appleOS = false
 #endif
 
+let isStaticBuild = Context.environment["SWIFTBUILD_STATIC_LINK"] != nil
 let useLocalDependencies = Context.environment["SWIFTCI_USE_LOCAL_DEPS"] != nil
 let useLLBuildFramework = Context.environment["SWIFTBUILD_LLBUILD_FWK"] != nil
 
@@ -447,6 +448,12 @@ for target in package.targets {
     }
 }
 
+if isStaticBuild {
+    package.targets = package.targets.filter { target in
+        target.type != .test && !target.name.hasSuffix("TestSupport")
+    }
+}
+
 // `SWIFTCI_USE_LOCAL_DEPS` configures if dependencies are locally available to build
 if useLocalDependencies {
     package.dependencies += [