[ORC] Break up OrcJIT library, add Orc-RPC based remote TargetProcessControl

implementation.

This patch aims to improve support for out-of-process JITing using OrcV2. It
introduces two new class templates, OrcRPCTargetProcessControlBase and
OrcRPCTPCServer, which together implement the TargetProcessControl API by
forwarding operations to an execution process via an Orc-RPC Endpoint. These
utilities are used to implement out-of-process JITing from llvm-jitlink to
a new llvm-jitlink-executor tool.

This patch also breaks the OrcJIT library into three parts:
  -- OrcTargetProcess: Contains code needed by the JIT execution process.
  -- OrcShared: Contains code needed by the JIT execution and compiler
     processes
  -- OrcJIT: Everything else.

This break-up allows JIT executor processes to link against OrcTargetProcess
and OrcShared only, without having to link in all of OrcJIT. Clients executing
JIT'd code in-process should start linking against OrcTargetProcess as well as
OrcJIT.

In the near future these changes will enable:
  -- Removal of the OrcRemoteTargetClient/OrcRemoteTargetServer class templates
     which provided similar functionality in OrcV1.
  -- Restoration of Chapter 5 of the Building-A-JIT tutorial series, which will
     serve as a simple usage example for these APIs.
  -- Implementation of lazy, cross-target compilation in lli's -jit-kind=orc-lazy
     mode.

Author: lhames

llvm/examples/OrcV2Examples/LLJITWithTargetProcessControl/LLJITWithTargetProcessControl.cpp

@@ -134,7 +134,8 @@ int main(int argc, char *argv[]) {
   ExitOnErr.setBanner(std::string(argv[0]) + ": ");
 
   // (1) Create LLJIT instance.
-  auto TPC = ExitOnErr(SelfTargetProcessControl::Create());
+  auto SSP = std::make_shared<SymbolStringPool>();
+  auto TPC = ExitOnErr(SelfTargetProcessControl::Create(std::move(SSP)));
   auto J = ExitOnErr(LLJITBuilder().setTargetProcessControl(*TPC).create());
 
   // (2) Install transform to print modules as they are compiled:

llvm/include/llvm/ExecutionEngine/JITLink/EHFrameSupport.h

@@ -21,14 +21,6 @@
 namespace llvm {
 namespace jitlink {
 
-/// Registers all FDEs in the given eh-frame section with the current process.
-Error registerEHFrameSection(const void *EHFrameSectionAddr,
-                             size_t EHFrameSectionSize);
-
-/// Deregisters all FDEs in the given eh-frame section with the current process.
-Error deregisterEHFrameSection(const void *EHFrameSectionAddr,
-                               size_t EHFrameSectionSize);
-
 /// Supports registration/deregistration of EH-frames in a target process.
 class EHFrameRegistrar {
 public:

llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h

@@ -801,6 +801,17 @@ class LinkGraph {
   /// Returns the endianness of content in this graph.
   support::endianness getEndianness() const { return Endianness; }
 
+  /// Allocate a copy of the given String using the LinkGraph's allocator.
+  /// This can be useful when renaming symbols or adding new content to the
+  /// graph.
+  StringRef allocateString(Twine Source) {
+    SmallString<256> TmpBuffer;
+    auto SourceStr = Source.toStringRef(TmpBuffer);
+    auto *AllocatedBuffer = Allocator.Allocate<char>(SourceStr.size());
+    llvm::copy(SourceStr, AllocatedBuffer);
+    return StringRef(AllocatedBuffer, SourceStr.size());
+  }
+
   /// Create a section with the given name, protection flags, and alignment.
   Section &createSection(StringRef Name, sys::Memory::ProtectionFlags Prot) {
     std::unique_ptr<Section> Sec(new Section(Name, Prot, Sections.size()));

llvm/include/llvm/ExecutionEngine/Orc/ExecutionUtils.h

@@ -41,17 +41,6 @@ namespace orc {
 
 class ObjectLayer;
 
-/// Run a main function, returning the result.
-///
-/// If the optional ProgramName argument is given then it will be inserted
-/// before the strings in Args as the first argument to the called function.
-///
-/// It is legal to have an empty argument list and no program name, however
-/// many main functions will expect a name argument at least, and will fail
-/// if none is provided.
-int runAsMain(int (*Main)(int, char *[]), ArrayRef<std::string> Args,
-              Optional<StringRef> ProgramName = None);
-
 /// This iterator provides a convenient way to iterate over the elements
 ///        of an llvm.global_ctors/llvm.global_dtors instance.
 ///