IR: Stop explicitly clearing the MDStringCache

The MDStringCache doesn't need to be explicitly cleared before
destruction.  The destructor handles it at least as efficiently.

llvm-svn: 265576

lib/IR/LLVMContextImpl.cpp

@@ -158,9 +158,6 @@ LLVMContextImpl::~LLVMContextImpl() {
   // Destroy ValuesAsMetadata.
   for (auto &Pair : ValuesAsMetadata)
     delete Pair.second;
-
-  // Destroy MDStrings.
-  MDStringCache.clear();
 }
 
 void LLVMContextImpl::dropTriviallyDeadConstantArrays() {