Use a sorted array instead of a map to store AttrBuilder string attributes

Using and std::map<SmallString, SmallString> for target dependent attributes is
inefficient: it makes its constructor slightly heavier, and involves extra
allocation for each new string attribute. Storing the attribute key/value as
strings implies extra allocation/copy step.

Use a sorted vector instead. Given the low number of attributes generally
involved, this is cheaper, as showcased by

https://llvm-compile-time-tracker.com/compare.php?from=5de322295f4ade692dc4f1823ae4450ad3c48af2&to=05bc480bf641a9e3b466619af43a2d123ee3f71d&stat=instructions

Differential Revision: https://reviews.llvm.org/D116599

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1349,7 +1349,7 @@ Error BitcodeReader::parseAttributeBlock() {
         return error("Invalid record");
 
       for (unsigned i = 0, e = Record.size(); i != e; i += 2) {
-        AttrBuilder B;
+        AttrBuilder B(Context);
         decodeLLVMAttributesForBitcode(B, Record[i+1]);
         Attrs.push_back(AttributeList::get(Context, Record[i], B));
       }
@@ -1591,7 +1591,7 @@ Error BitcodeReader::parseAttributeGroupBlock() {
       uint64_t GrpID = Record[0];
       uint64_t Idx = Record[1]; // Index of the object this attribute refers to.
 
-      AttrBuilder B;
+      AttrBuilder B(Context);
       for (unsigned i = 2, e = Record.size(); i != e; ++i) {
         if (Record[i] == 0) {        // Enum attribute
           Attribute::AttrKind Kind;