[ORC] Add support for emulated TLS to ORCv2.

This commit adds a ManglingOptions struct to IRMaterializationUnit, and replaces
IRCompileLayer::CompileFunction with a new IRCompileLayer::IRCompiler class. The
ManglingOptions struct defines the emulated-TLS state (via a bool member,
EmulatedTLS, which is true if emulated-TLS is enabled and false otherwise). The
IRCompileLayer::IRCompiler class wraps an IRCompiler (the same way that the
CompileFunction typedef used to), but adds a method to return the
IRCompileLayer::ManglingOptions that the compiler will use.

These changes allow us to correctly determine the symbols that will be produced
when a thread local global variable defined at the IR level is compiled with or
without emulated TLS. This is required for ORCv2, where MaterializationUnits
must declare their interface up-front.

Most ORCv2 clients should not require any changes. Clients writing custom IR
compilers will need to wrap their compiler in an IRCompileLayer::IRCompiler,
rather than an IRCompileLayer::CompileFunction, however this should be a
straightforward change (see modifications to CompileUtils.* in this patch for an
example).
diff --git a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
index 4ffcade..f81e584 100644
--- a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
@@ -107,7 +107,7 @@
   return std::unique_ptr<ObjectLayer>(std::move(ObjLinkingLayer));
 }
 
-Expected<IRCompileLayer::CompileFunction>
+Expected<std::unique_ptr<IRCompileLayer::IRCompiler>>
 LLJIT::createCompileFunction(LLJITBuilderState &S,
                              JITTargetMachineBuilder JTMB) {
 
@@ -118,13 +118,13 @@
   // Otherwise default to creating a SimpleCompiler, or ConcurrentIRCompiler,
   // depending on the number of threads requested.
   if (S.NumCompileThreads > 0)
-    return ConcurrentIRCompiler(std::move(JTMB));
+    return std::make_unique<ConcurrentIRCompiler>(std::move(JTMB));
 
   auto TM = JTMB.createTargetMachine();
   if (!TM)
     return TM.takeError();
 
-  return TMOwningSimpleCompiler(std::move(*TM));
+  return std::make_unique<TMOwningSimpleCompiler>(std::move(*TM));
 }
 
 LLJIT::LLJIT(LLJITBuilderState &S, Error &Err)