[LangRef] Use new memory attribute syntax (NFC)

Use the memory() spelling in a few places that were using the
old syntax.

The documented attributes for llvm.type.checked.load don't match
the actual attributes, I've raised this here:
https://reviews.llvm.org/D21121#inline-1406792

llvm/docs/LangRef.rst

@@ -2474,8 +2474,8 @@ operand bundle to not miscompile programs containing it.
 - The bundle operands for an unknown operand bundle escape in unknown
   ways before control is transferred to the callee or invokee.
 - Calls and invokes with operand bundles have unknown read / write
-  effect on the heap on entry and exit (even if the call target is
-  ``readnone`` or ``readonly``), unless they're overridden with
+  effect on the heap on entry and exit (even if the call target specifies
+  a ``memory`` attribute), unless they're overridden with
   callsite specific attributes.
 - An operand bundle at a call site cannot change the implementation
   of the called function.  Inter-procedural optimizations work as
@@ -10758,14 +10758,14 @@ to LLVM:
       return &s[1].Z.B[5][13];
     }
 
-The LLVM code generated by Clang is:
+The LLVM code generated by Clang is approximately:
 
 .. code-block:: llvm
 
     %struct.RT = type { i8, [10 x [20 x i32]], i8 }
     %struct.ST = type { i32, double, %struct.RT }
 
-    define ptr @foo(ptr %s) nounwind uwtable readnone optsize ssp {
+    define ptr @foo(ptr %s) {
     entry:
       %arrayidx = getelementptr inbounds %struct.ST, ptr %s, i64 1, i32 2, i32 1, i64 5, i64 13
       ret ptr %arrayidx
@@ -12885,7 +12885,7 @@ Syntax:
       declare <pointer type>
         @llvm.experimental.gc.get.pointer.base(
           <pointer type> readnone nocapture %derived_ptr)
-          nounwind readnone willreturn
+          nounwind willreturn memory(none)
 
 Overview:
 """""""""
@@ -12923,7 +12923,7 @@ Syntax:
       declare i64
         @llvm.experimental.gc.get.pointer.offset(
           <pointer type> readnone nocapture %derived_ptr)
-          nounwind readnone willreturn
+          nounwind willreturn memory(none)
 
 Overview:
 """""""""
@@ -25592,7 +25592,7 @@ Syntax:
 
 ::
 
-      declare type @llvm.ssa.copy(type %operand) returned(1) readnone
+      declare type @llvm.ssa.copy(type returned %operand) memory(none)
 
 Arguments:
 """"""""""
@@ -25619,7 +25619,7 @@ Syntax:
 
 ::
 
-      declare i1 @llvm.type.test(ptr %ptr, metadata %type) nounwind readnone
+      declare i1 @llvm.type.test(ptr %ptr, metadata %type) nounwind memory(none)
 
 
 Arguments:
@@ -25644,7 +25644,7 @@ Syntax:
 
 ::
 
-      declare {ptr, i1} @llvm.type.checked.load(ptr %ptr, i32 %offset, metadata %type) argmemonly nounwind readonly
+      declare {ptr, i1} @llvm.type.checked.load(ptr %ptr, i32 %offset, metadata %type) nounwind memory(argmem: read)
 
 
 Arguments:
@@ -25734,7 +25734,7 @@ Syntax:
 
 ::
 
-      declare void @llvm.donothing() nounwind readnone
+      declare void @llvm.donothing() nounwind memory(none)
 
 Overview:
 """""""""
@@ -26030,7 +26030,7 @@ Syntax:
 
 ::
 
-      declare ptr @llvm.load.relative.iN(ptr %ptr, iN %offset) argmemonly nounwind readonly
+      declare ptr @llvm.load.relative.iN(ptr %ptr, iN %offset) nounwind memory(argmem: read)
 
 Overview:
 """""""""
@@ -26089,9 +26089,9 @@ This is an overloaded intrinsic. You can use llvm.is.constant with any argument
 
 ::
 
-      declare i1 @llvm.is.constant.i32(i32 %operand) nounwind readnone
-      declare i1 @llvm.is.constant.f32(float %operand) nounwind readnone
-      declare i1 @llvm.is.constant.TYPENAME(TYPE %operand) nounwind readnone
+      declare i1 @llvm.is.constant.i32(i32 %operand) nounwind memory(none)
+      declare i1 @llvm.is.constant.f32(float %operand) nounwind memory(none)
+      declare i1 @llvm.is.constant.TYPENAME(TYPE %operand) nounwind memory(none)
 
 Overview:
 """""""""
@@ -26134,7 +26134,7 @@ Syntax:
 
 ::
 
-      declare ptrty llvm.ptrmask(ptrty %ptr, intty %mask) readnone speculatable
+      declare ptrty llvm.ptrmask(ptrty %ptr, intty %mask) speculatable memory(none)
 
 Arguments:
 """"""""""
@@ -26170,7 +26170,7 @@ Syntax:
 
 ::
 
-      declare ptr @llvm.threadlocal.address(ptr) nounwind readnone willreturn
+      declare ptr @llvm.threadlocal.address(ptr) nounwind willreturn memory(none)
 
 Arguments:
 """"""""""