LLVM11 related build fix

Explicit `std::string` cast and CallGraph #include.

Author: pszymich

IGC/Compiler/FixInvalidFuncNamePass.cpp

@@ -52,7 +52,7 @@ bool FixInvalidFuncName::runOnFunction(Function& F)
                 if (func)
                 {
                     StringRef original = func->getName();
-                    std::string changed = replaceInvalidCharToUnderline(original);
+                    std::string changed = replaceInvalidCharToUnderline(original.str());
                     if (original != changed)
                     {
                         func->setName(changed);

IGC/VectorCompiler/lib/GenXCodeGen/GenXStackUsage.cpp

@@ -20,6 +20,7 @@ SPDX-License-Identifier: MIT
 #include "vc/Support/GenXDiagnostic.h"
 #include "vc/Support/BackendConfig.h"
 
+#include <llvm/Analysis/CallGraph.h>
 #include <llvm/CodeGen/TargetPassConfig.h>
 #include <llvm/GenXIntrinsics/GenXMetadata.h>
 #include <llvm/IR/Instructions.h>