Driver: emit a diagnostic if clang++ is not found

Rather than aborting due to an assertion failure, emit a diagnostic.
This is much safer and generally easier to understand why the command
failed.  It solves the problem of running swiftc from the build without
the path being set such that the clang++ driver is found by the swift
driver.

Author: compnerd

lib/Driver/WindowsToolChains.cpp

@@ -80,7 +80,7 @@ toolchains::Windows::constructInvocation(const DynamicLinkJobAction &job,
 
   // Configure the toolchain.
   // By default, use the system clang++ to link.
-  const char *Clang = nullptr;
+  const char *Clang = "clang++";
   if (const Arg *A = context.Args.getLastArg(options::OPT_tools_directory)) {
     StringRef toolchainPath(A->getValue());
 
@@ -89,12 +89,6 @@ toolchains::Windows::constructInvocation(const DynamicLinkJobAction &job,
             llvm::sys::findProgramByName("clang++", {toolchainPath}))
       Clang = context.Args.MakeArgString(toolchainClang.get());
   }
-  if (Clang == nullptr) {
-    if (auto pathClang = llvm::sys::findProgramByName("clang++", None))
-      Clang = context.Args.MakeArgString(pathClang.get());
-  }
-  assert(Clang &&
-         "clang++ was not found in the toolchain directory or system path.");
 
   std::string Target = getTriple().str();
   if (!Target.empty()) {

test/Driver/windows-link-job.swift

@@ -0,0 +1,2 @@
+// RUN: env PATH= %swiftc_driver_plain -target x86_64-unknown-windows-msvc -### -module-name link -emit-library %s 2>&1 | %FileCheck %s
+// CHECK: {{^}}clang++