Add utility to check if LLDB is present, to skip tests that rely on LLDB

The driver very well may be built and tested in environments that do not have LLDB.
We should skip tests that rely on LLDB being present in such environments.

Author: artemcm

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -18,6 +18,27 @@ import XCTest
 
 final class SwiftDriverTests: XCTestCase {
 
+  /// Determine if the test's execution environment has LLDB
+  /// Used to skip tests that rely on LLDB in such environments.
+  private func testEnvHasLLDB() throws -> Bool {
+    let executor = try SwiftDriverExecutor(diagnosticsEngine: DiagnosticsEngine(),
+                                           processSet: ProcessSet(),
+                                           fileSystem: localFileSystem,
+                                           env: ProcessEnv.vars)
+    let toolchain: Toolchain
+    #if os(macOS)
+    toolchain = DarwinToolchain(env: ProcessEnv.vars, executor: executor)
+    #else
+    toolchain = GenericUnixToolchain(env: ProcessEnv.vars, executor: executor)
+    #endif
+    do {
+      _ = try toolchain.getToolPath(.lldb)
+    } catch ToolchainError.unableToFind {
+      return false
+    }
+    return true
+  }
+
   func testInvocationRunModes() throws {
 
     let driver1 = try Driver.invocationRunMode(forArgs: ["swift"])
@@ -1631,20 +1652,8 @@ final class SwiftDriverTests: XCTestCase {
 
   func testRepl() throws {
     // Do not run this test if no LLDB is found in the toolchain.
-    let executor = try SwiftDriverExecutor(diagnosticsEngine: DiagnosticsEngine(),
-                                           processSet: ProcessSet(),
-                                           fileSystem: localFileSystem,
-                                           env: ProcessEnv.vars)
-    var toolchain: Toolchain
-    #if os(macOS)
-    toolchain = DarwinToolchain(env: ProcessEnv.vars, executor: executor)
-    #else
-    toolchain = GenericUnixToolchain(env: ProcessEnv.vars, executor: executor)
-    #endif
-    do {
-      _ = try toolchain.getToolPath(.lldb)
-    } catch ToolchainError.unableToFind {
-      return
+    if try !testEnvHasLLDB() {
+      throw XCTSkip()
     }
 
     func isLLDBREPLFlag(_ arg: Job.ArgTemplate) -> Bool {
@@ -2618,9 +2627,12 @@ final class SwiftDriverTests: XCTestCase {
   }
 
   func testNoInputs() throws {
-    do {
-      var driver = try Driver(args: ["swift"])
-      XCTAssertNoThrow(try driver.planBuild())
+    // A plain `swift` invocation requires lldb to be present
+    if try testEnvHasLLDB() {
+      do {
+        var driver = try Driver(args: ["swift"])
+        XCTAssertNoThrow(try driver.planBuild())
+      }
     }
     do {
       var driver = try Driver(args: ["swiftc"])