[llvm-mca] Add HardwareUnit and Context classes.

This patch moves the construction of the default backend from llvm-mca.cpp and
into mca::Context. The Context class is responsible for holding ownership of
the simulated hardware components. These components are subclasses of
HardwareUnit. Right now the HardwareUnit is pretty bare-bones, but eventually
we might want to add some common functionality across all hardware components,
such as isReady() or something similar.

I have a feeling this patch will probably need some updates, but it's a start.
One thing I am not particularly fond of is the rather large interface for
createDefaultPipeline. That convenience routine takes a rather large set of
inputs from the llvm-mca driver, where many of those inputs are generated via
command line options.

One item I think we might want to change is the separating of ownership of
hardware components (owned by the context) and the pipeline (which owns
Stages). In short, a Pipeline owns Stages, a Context (currently) owns hardware.
The Pipeline's Stages make use of the components, and thus there is a lifetime
dependency generated. The components must outlive the pipeline. We could solve
this by having the Context also own the Pipeline, and not return a
unique_ptr<Pipeline>. Now that I think about it, I like that idea more.

Differential Revision: https://reviews.llvm.org/D48691

llvm-svn: 336456
diff --git a/llvm/tools/llvm-mca/HardwareUnit.h b/llvm/tools/llvm-mca/HardwareUnit.h
new file mode 100644
index 0000000..e8c496a
--- /dev/null
+++ b/llvm/tools/llvm-mca/HardwareUnit.h
@@ -0,0 +1,31 @@
+//===-------------------------- HardwareUnit.h ------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+/// \file
+///
+/// This file defines a base class for describing a simulated hardware
+/// unit.  These units are used to construct a simulated backend.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TOOLS_LLVM_MCA_HARDWAREUNIT_H
+#define LLVM_TOOLS_LLVM_MCA_HARDWAREUNIT_H
+
+namespace mca {
+
+class HardwareUnit {
+  HardwareUnit(const HardwareUnit &H) = delete;
+  HardwareUnit &operator=(const HardwareUnit &H) = delete;
+
+public:
+  HardwareUnit() = default;
+  virtual ~HardwareUnit();
+};
+
+} // namespace mca
+#endif // LLVM_TOOLS_LLVM_MCA_HARDWAREUNIT_H