[IR] Adds mustprogress as a LLVM IR attribute

This adds the LLVM IR attribute `mustprogress` as defined in LangRef through D86233. This attribute will be applied to functions with in languages like C++ where forward progress is guaranteed. Functions without this attribute are not required to make progress.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D85393

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1535,6 +1535,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoUndef;
   case bitc::ATTR_KIND_BYREF:
     return Attribute::ByRef;
+  case bitc::ATTR_KIND_MUSTPROGRESS:
+    return Attribute::MustProgress;
   }
 }