Fix warnings as errors (building with Clang) (#155)

While building the project with more warnings-as-errors, I came across
these two ones. See comments inline.

Author: jgsogo

CMakeLists.txt

@@ -91,6 +91,7 @@ message(STATUS "QSS Compiler Version: ${QSSC_VERSION}")
 
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+set(CMAKE_COMPILE_WARNING_AS_ERROR ON)
 
 # Compiler options
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")

include/Utils/DebugIndent.h

@@ -56,7 +56,14 @@ class DebugIndent {
 
 private:
   unsigned int debugIndentCount{0};
-  unsigned int debugIndentStep{2};
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wattributes"
+#endif
+  [[maybe_unused]] unsigned int debugIndentStep{2};
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
 };
 
 } // namespace qssc::utils

lib/Frontend/OpenQASM3/QUIRVariableBuilder.cpp

@@ -130,8 +130,8 @@ QUIRVariableBuilder::generateParameterLoad(mlir::Location location,
     return loadOp;
   }
 
-  assert(false &&
-         "Unsupported defining value operation for parameter variable");
+  llvm_unreachable(
+      "Unsupported defining value operation for parameter variable");
 }
 
 void QUIRVariableBuilder::generateArrayVariableDeclaration(