[IR] Add interface to remove a CallBase string function attribute

[tejohnson]

Summary

Adds an interface to remove a string function attribute attached to a
CallBase, and a corresponding unittest.

This was extracted from D141077, and will be used by a follow on patch
that removes memprof attributes when needed.

[snehasish]

lgtm

llvm/unittests/IR/InstructionsTest.cpp

107

ASSERT_TRUE since it doesn't make sense to continue if we fail here.

[tejohnson]

Address comment

llvm/include/llvm/IR/InstrTypes.h

void removeFnAttrs(const AttributeMask &AttrsToRemove) {
     Attrs = Attrs.removeFnAttributes(getContext(), AttrsToRemove);
   }
 
   /// Removes the attribute from the function
   void removeFnAttr(Attribute::AttrKind Kind) {
     Attrs = Attrs.removeFnAttribute(getContext(), Kind);
   }
 
+  /// Removes the attribute from the function
+  void removeFnAttr(StringRef Kind) {
+    Attrs = Attrs.removeFnAttribute(getContext(), Kind);
+  }
+
   /// Removes the attribute from the return value
   void removeRetAttr(Attribute::AttrKind Kind) {
     Attrs = Attrs.removeRetAttribute(getContext(), Kind);
   }
 
   /// Removes the attributes from the return value
   void removeRetAttrs(const AttributeMask &AttrsToRemove) {
     Attrs = Attrs.removeRetAttributes(getContext(), AttrsToRemove);

llvm/unittests/IR/InstructionsTest.cpp

for (Value *Arg : Call->args()) {
     EXPECT_EQ(FArgTypes[Idx], Arg->getType());
     EXPECT_EQ(Call->getArgOperand(Idx)->getType(), Arg->getType());
     Idx++;
   }
 
   Call->addRetAttr(Attribute::get(Call->getContext(), "test-str-attr"));
   EXPECT_TRUE(Call->hasRetAttr("test-str-attr"));
   EXPECT_FALSE(Call->hasRetAttr("not-on-call"));
+
+  Call->addFnAttr(Attribute::get(Call->getContext(), "test-str-fn-attr"));
+  EXPECT_TRUE(Call->hasFnAttr("test-str-fn-attr"));
+  Call->removeFnAttr("test-str-fn-attr");
+  EXPECT_FALSE(Call->hasFnAttr("test-str-fn-attr"));
 }
 
 TEST_F(ModuleWithFunctionTest, InvokeInst) {
   BasicBlock *BB1 = BasicBlock::Create(Ctx, "", F);
   BasicBlock *BB2 = BasicBlock::Create(Ctx, "", F);
 
   Value *Args[] = {ConstantInt::get(Type::getInt8Ty(Ctx), 20),
                    ConstantInt::get(Type::getInt32Ty(Ctx), 9999),