Drop 'using namespace std' everywhere (#2755)

Most uses of std functionality already have explicit `std::`
qualifications, so qualify the remaining unqualified uses and drop the
`using namespace` directives.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@29b02642964a240

Author: svenvh

llvm-spirv/lib/SPIRV/SPIRVReader.cpp

@@ -94,7 +94,6 @@
 
 #define DEBUG_TYPE "spirv"
 
-using namespace std;
 using namespace llvm;
 using namespace SPIRV;
 using namespace OCLUtil;
@@ -1258,8 +1257,8 @@ Value *SPIRVToLLVM::oclTransConstantSampler(SPIRV::SPIRVConstantSampler *BCS,
 Value *SPIRVToLLVM::oclTransConstantPipeStorage(
     SPIRV::SPIRVConstantPipeStorage *BCPS) {
 
-  string CPSName = string(kSPIRVTypeName::PrefixAndDelim) +
-                   kSPIRVTypeName::ConstantPipeStorage;
+  std::string CPSName = std::string(kSPIRVTypeName::PrefixAndDelim) +
+                        kSPIRVTypeName::ConstantPipeStorage;
 
   auto *Int32Ty = IntegerType::getInt32Ty(*Context);
   auto *CPSTy = StructType::getTypeByName(*Context, CPSName);
@@ -3439,7 +3438,7 @@ SPIRVToLLVM::SPIRVToLLVM(Module *LLVMModule, SPIRVModule *TheSPIRVModule)
 }
 
 std::string getSPIRVFuncSuffix(SPIRVInstruction *BI) {
-  string Suffix = "";
+  std::string Suffix = "";
   if (BI->getOpCode() == OpCreatePipeFromPipeStorage) {
     auto *CPFPS = static_cast<SPIRVCreatePipeFromPipeStorage *>(BI);
     assert(CPFPS->getType()->isTypePipe() &&
@@ -3495,7 +3494,7 @@ std::string getSPIRVFuncSuffix(SPIRVInstruction *BI) {
     int Dim = EleTy->isTypeArray() ? EleTy->getArrayLength() : 1;
     assert((EleTy->isTypeInt() && Dim == 1) ||
            (EleTy->isTypeArray() && Dim >= 2 && Dim <= 3));
-    ostringstream OS;
+    std::ostringstream OS;
     OS << Dim;
     Suffix += OS.str() + "D";
   }

llvm-spirv/lib/SPIRV/SPIRVToLLVMDbgTran.cpp

@@ -51,7 +51,6 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
 
-using namespace std;
 using namespace SPIRVDebug::Operand;
 
 namespace SPIRV {
@@ -1806,7 +1805,7 @@ DIBuilder &SPIRVToLLVMDbgTran::getDIBuilder(const SPIRVExtInst *DebugInst) {
   return *BuilderMap[DebugInst->getId()];
 }
 
-SPIRVToLLVMDbgTran::SplitFileName::SplitFileName(const string &FileName) {
+SPIRVToLLVMDbgTran::SplitFileName::SplitFileName(const std::string &FileName) {
   auto Loc = FileName.find_last_of("/\\");
   if (Loc != std::string::npos) {
     BaseName = FileName.substr(Loc + 1);
@@ -1835,7 +1834,7 @@ SPIRVToLLVMDbgTran::ParseChecksum(StringRef Text) {
   auto KindPos = Text.find(SPIRVDebug::ChecksumKindPrefx);
   if (KindPos != StringRef::npos) {
     auto ColonPos = Text.find(":", KindPos);
-    KindPos += string("//__").size();
+    KindPos += std::string("//__").size();
     auto KindStr = Text.substr(KindPos, ColonPos - KindPos);
     auto Checksum = Text.substr(ColonPos).ltrim(':');
     if (auto Kind = DIFile::getChecksumKind(KindStr)) {