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.

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.
+ */
+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).

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
+    explicit Module(UPModule&& module, std::string const& entrypoint);
+
     // Construct with an LLVM IR file (bitcode or disassembled)
     explicit Module(std::string const& filename);