[NFC][STLExtras] Add make_first_range(), similar to existing make_second_range()

Having just one of the two seens weird.
I wanted to use it a few times, but it wasn't there.

Author: LebedevRI

llvm/include/llvm/ADT/STLExtras.h

@@ -1237,6 +1237,15 @@ class indexed_accessor_range
   }
 };
 
+/// Given a container of pairs, return a range over the first elements.
+template <typename ContainerTy> auto make_first_range(ContainerTy &&c) {
+  return llvm::map_range(
+      std::forward<ContainerTy>(c),
+      [](decltype((*std::begin(c))) elt) -> decltype((elt.first)) {
+        return elt.first;
+      });
+}
+
 /// Given a container of pairs, return a range over the second elements.
 template <typename ContainerTy> auto make_second_range(ContainerTy &&c) {
   return llvm::map_range(