Allow construction from module and specific entry point name (#5)

* Add constructor for llvm::Module + entrypoint name

This is useful when a user generates an llvm::Module that may have
multiple entry points, as the Qwerty compiler/runtime does.

* XaccQuantum: Add setter for accelerator/shots

In my experience, it has been easier to treat XaccQuantum as a singleton
to avoid issues with XACC initializing itself twice.

It is probably cleaner to pass (accelerator, shots) via qiree::Executor,
but this solves my immediate implementaiton problem in the Qwerty
compiler/runtime.

* Address PR comments

* Address PR comments

src/qiree/Module.cc

@@ -113,6 +113,22 @@ Module::Module(UPModule&& module) : module_{std::move(module)}
                    << module_->getSourceFileName() << "'");
 }
 
+//---------------------------------------------------------------------------//
+/*!
+ * Construct with an LLVM module and an entry point.
+ * Useful when there are multiple entry points.
+ */
+Module::Module(UPModule&& module, std::string const& entrypoint)
+    : module_{std::move(module)}
+{
+    QIREE_EXPECT(module_);
+
+    // Search for explicitly named entry point
+    entrypoint_ = module_->getFunction(entrypoint);
+    QIREE_VALIDATE(entrypoint_,
+                   << "no entrypoint function '" << entrypoint << "' exists");
+}
+
 //---------------------------------------------------------------------------//
 /*!
  * Construct with an LLVM IR file (bitcode or disassembled).
@@ -125,6 +141,7 @@ Module::Module(std::string const& filename)
 //---------------------------------------------------------------------------//
 /*!
  * Construct with an LLVM IR file (bitcode or disassembled) and entry point.
+ * Useful when there are multiple entry points.
  */
 Module::Module(std::string const& filename, std::string const& entrypoint)
     : module_{load_llvm_module(filename)}

src/qiree/Module.hh

@@ -45,6 +45,9 @@ class Module
     // Construct from an externally created LLVM module
     explicit Module(UPModule&& module);
 
+    // Construct from an externally created LLVM module and an entry point
+    Module(UPModule&& module, std::string const& entrypoint);
+
     // Construct with an LLVM IR file (bitcode or disassembled)
     explicit Module(std::string const& filename);