[Driver] On Apple platforms, add /usr/lib/swift to the beginning of the DYLD_LIBRARY_PATH value in constructInvocation.

rdar://problem/47819780

Author: mikeash

lib/Driver/DarwinToolChains.cpp

@@ -38,6 +38,10 @@ using namespace swift;
 using namespace swift::driver;
 using namespace llvm::opt;
 
+/// The path for Swift libraries in the OS. (Duplicated from Immediate.cpp.
+/// Eventually we should consolidate this.)
+#define OS_LIBRARY_PATH "/usr/lib/swift"
+
 std::string
 toolchains::Darwin::findProgramRelativeToSwiftImpl(StringRef name) const {
   StringRef swiftPath = getDriver().getSwiftProgramPath();
@@ -71,12 +75,15 @@ toolchains::Darwin::constructInvocation(const InterpretJobAction &job,
                                         const JobContext &context) const {
   InvocationInfo II = ToolChain::constructInvocation(job, context);
 
+  SmallString<128> envValue(OS_LIBRARY_PATH ":");
+
   SmallString<128> runtimeLibraryPath;
   getRuntimeLibraryPath(runtimeLibraryPath, context.Args, /*Shared=*/true);
+  envValue.append(runtimeLibraryPath);
 
   addPathEnvironmentVariableIfNeeded(II.ExtraEnvironment, "DYLD_LIBRARY_PATH",
                                      ":", options::OPT_L, context.Args,
-                                     runtimeLibraryPath);
+                                     envValue);
   addPathEnvironmentVariableIfNeeded(II.ExtraEnvironment, "DYLD_FRAMEWORK_PATH",
                                      ":", options::OPT_F, context.Args);
   // FIXME: Add options::OPT_Fsystem paths to DYLD_FRAMEWORK_PATH as well.

test/Driver/environment-mac.swift

@@ -0,0 +1,5 @@
+// REQUIRES: OS=macosx
+
+// RUN: %swift_driver -driver-use-frontend-path %S/Inputs/print-var.sh %s DYLD_LIBRARY_PATH | %FileCheck %s
+
+// CHECK: {{^/usr/lib/swift:}}