[NFC] Use SmallString instead of std::string for the AttrBuilder

[serge-sans-paille]

Summary

This avoids a few unnecessary conversion from StringRef to std::string, and a
bunch of extra allocation thanks to the SmallString.

[bkramer]

Do we have some information on how long these attributes typically are? libc++'s std::string has an inline capacity of 22, which is why I've been reluctant to change this so far.

[serge-sans-paille]

In D98190#2613701, @bkramer wrote:

Do we have some information on how long these attributes typically are? libc++'s std::string has an inline capacity of 22, which is why I've been reluctant to change this so far.

Here is what I know fore sure:

When parsing the SQLite amalgametion and counting the number of instructions, which is a fairly stable metric:

perf stat ./bin/clang -w -c sqlite3.c -o/dev/null

I get 6,005,593,103 instructions *before* that patch and 5,974,383,558 instructions *after* the patch.

[bkramer]

Sounds like a good enough improvement to me, thanks for checking.

llvm/include/llvm/IR/Attributes.h

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_IR_ATTRIBUTES_H
 #define LLVM_IR_ATTRIBUTES_H
 
 #include "llvm-c/Types.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Alignment.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
 #include <bitset>
 #include <cassert>
 #include <cstdint>

 //===----------------------------------------------------------------------===//
 /// \class
 /// This class is used in conjunction with the Attribute::get method to
 /// create an Attribute object. The object itself is uniquified. The Builder's
 /// value, however, is not. So this can be used as a quick way to test for
 /// equality, presence of attributes, etc.
 class AttrBuilder {
   std::bitset<Attribute::EndAttrKinds> Attrs;
-  std::map<std::string, std::string, std::less<>> TargetDepAttrs;
+  std::map<SmallString<32>, SmallString<32>, std::less<>> TargetDepAttrs;
   MaybeAlign Alignment;
   MaybeAlign StackAlignment;
   uint64_t DerefBytes = 0;
   uint64_t DerefOrNullBytes = 0;
   uint64_t AllocSizeArgs = 0;
   Type *ByValType = nullptr;
   Type *StructRetType = nullptr;
   Type *ByRefType = nullptr;
public:
   /// Attribute.getIntValue().
   AttrBuilder &addAllocSizeAttrFromRawRepr(uint64_t RawAllocSizeRepr);
 
   /// Return true if the builder contains no target-independent
   /// attributes.
   bool empty() const { return Attrs.none(); }
 
   // Iterators for target-dependent attributes.
-  using td_type = std::pair<std::string, std::string>;
+  using td_type = decltype(TargetDepAttrs)::value_type;
   using td_iterator = decltype(TargetDepAttrs)::iterator;
   using td_const_iterator = decltype(TargetDepAttrs)::const_iterator;
   using td_range = iterator_range<td_iterator>;
   using td_const_range = iterator_range<td_const_iterator>;
 
   td_iterator td_begin() { return TargetDepAttrs.begin(); }
   td_iterator td_end() { return TargetDepAttrs.end(); }
 

llvm/lib/IR/Attributes.cpp

AttrBuilder &AttrBuilder::addAttribute(Attribute Attr) {
   else if (Kind == Attribute::DereferenceableOrNull)
     DerefOrNullBytes = Attr.getDereferenceableOrNullBytes();
   else if (Kind == Attribute::AllocSize)
     AllocSizeArgs = Attr.getValueAsInt();
   return *this;
 }
 
 AttrBuilder &AttrBuilder::addAttribute(StringRef A, StringRef V) {
-  TargetDepAttrs[std::string(A)] = std::string(V);
+  TargetDepAttrs[A] = V;
   return *this;
 }
 
 AttrBuilder &AttrBuilder::removeAttribute(Attribute::AttrKind Val) {
   assert((unsigned)Val < Attribute::EndAttrKinds && "Attribute out of range!");
   Attrs[Val] = false;
 
   if (Val == Attribute::Alignment)