[RustDemangle] remove StringView::dropFront

[nickdesaulniers]

Summary

Toward the goal of replacing llvm::StringView with std::string_view,
first replacing users of llvm::StringView::dropFront, this case in the
Rust demangling scheme seemed worth its own commit+review.

[MaskRay]

s/StringView/StringRef/

[nickdesaulniers]

In D148272#4266497, @MaskRay wrote:

s/StringView/StringRef/

No; I mean to replace StringView entirely.

[nickdesaulniers]

• get rid of suffix entirely, skip substring if there's no .

[MaskRay]

OK. I didn't notice there is a StringView inside LLVMDemangle :(

llvm/lib/Demangle/RustDemangle.cpp

bool Demangler::demangle(StringView Mangled) {
   RecursionLevel = 0;
   BoundLifetimes = 0;
 
   if (!Mangled.consumeFront("_R")) {
     Error = true;
     return false;
   }
   size_t Dot = Mangled.find('.');
-  Input = Mangled.substr(0, Dot);
-  StringView Suffix = Mangled.dropFront(Dot);
+  Input = Dot == StringView::npos ? Mangled : Mangled.substr(0, Dot);
 
   demanglePath(IsInType::No);
 
   if (Position != Input.size()) {
     ScopedOverride<bool> SavePrint(Print, false);
     demanglePath(IsInType::No);
   }
 
   if (Position != Input.size())
     Error = true;
 
-  if (!Suffix.empty()) {
+  if (Dot != StringView::npos) {
     print(" (");
-    print(Suffix);
+    print(Mangled.substr(Dot));
     print(")");
   }
 
   return !Error;
 }
 
 // Demangles a path. InType indicates whether a path is inside a type. When
 // LeaveOpen is true, a closing `>` after generic arguments is omitted from the