Apply clang-tidy's modernize-loop-convert to most of lib/IR.

Only minor manual fixes. No functionality change intended.

llvm-svn: 273813
diff --git a/llvm/lib/IR/LLVMContextImpl.cpp b/llvm/lib/IR/LLVMContextImpl.cpp
index d27fcb1..3e354b5 100644
--- a/llvm/lib/IR/LLVMContextImpl.cpp
+++ b/llvm/lib/IR/LLVMContextImpl.cpp
@@ -99,10 +99,9 @@
   InlineAsms.freeConstants();
   DeleteContainerSeconds(IntConstants);
   DeleteContainerSeconds(FPConstants);
-  
-  for (StringMap<ConstantDataSequential*>::iterator I = CDSConstants.begin(),
-       E = CDSConstants.end(); I != E; ++I)
-    delete I->second;
+
+  for (auto &CDSConstant : CDSConstants)
+    delete CDSConstant.second;
   CDSConstants.clear();
 
   // Destroy attributes.