[RustDemangle] remove StringView::dropFront

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.

Reviewed By: erichkeane, MaskRay

Differential Revision: https://reviews.llvm.org/D148272

Author: nickdesaulniers

llvm/lib/Demangle/RustDemangle.cpp

@@ -202,8 +202,7 @@ bool Demangler::demangle(StringView Mangled) {
     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);
 
@@ -215,9 +214,9 @@ bool Demangler::demangle(StringView Mangled) {
   if (Position != Input.size())
     Error = true;
 
-  if (!Suffix.empty()) {
+  if (Dot != StringView::npos) {
     print(" (");
-    print(Suffix);
+    print(Mangled.substr(Dot));
     print(")");
   }