Cache LLVM attributes for intrinsics

Looking these up is slow because the attribute lists get rebuilt and
reuniqued in the LLVM context each time. Saving them is a single
pointer per IntrinsicInfo and IntrinsicInfos are already cached in
each SILModule.

This shaves about 4s off optimizing the standard library (72s out of a
3:00+ build), and probably has much less effect anywhere else. That
is, it's not /really/ important after all, but it was an easy bit of
the profile to hammer down.

Author: jrose-apple

include/swift/AST/Builtins.h

@@ -107,7 +107,10 @@ class BuiltinInfo {
 };
 
 /// \brief The information identifying the llvm intrinsic - its id and types.
-struct IntrinsicInfo {
+class IntrinsicInfo {
+  mutable llvm::AttributeList Attrs =
+      llvm::DenseMapInfo<llvm::AttributeList>::getEmptyKey();
+public:
   llvm::Intrinsic::ID ID;
   SmallVector<Type, 4> Types;
   bool hasAttribute(llvm::Attribute::AttrKind Kind) const;

lib/AST/Builtins.cpp

@@ -45,10 +45,12 @@ bool BuiltinInfo::isReadNone() const {
 }
 
 bool IntrinsicInfo::hasAttribute(llvm::Attribute::AttrKind Kind) const {
-  // FIXME: We should not be relying on the global LLVM context.
-  llvm::AttributeList attrs =
-      llvm::Intrinsic::getAttributes(getGlobalLLVMContext(), ID);
-  return (attrs.hasAttribute(llvm::AttributeList::FunctionIndex, Kind));
+  using DenseMapInfo = llvm::DenseMapInfo<llvm::AttributeList>;
+  if (DenseMapInfo::isEqual(Attrs, DenseMapInfo::getEmptyKey())) {
+    // FIXME: We should not be relying on the global LLVM context.
+    Attrs = llvm::Intrinsic::getAttributes(getGlobalLLVMContext(), ID);
+  }
+  return Attrs.hasFnAttribute(Kind);
 }
 
 Type swift::getBuiltinType(ASTContext &Context, StringRef Name) {