[Demangle] avoid more std::string_view::substr

[nickdesaulniers]

Summary

In D148959, I removed usage of std::string_view::substr because it may
throw, and libcxxabi cannot use such code. I missed one instance in
llvm::starts_with. That is blocking copying the code back upstream in
D148566.

Mark these helpers noexcept (as they are in C++20) as well, to remind
future travelers.

Make these changes upstream, and copy them back downstream using
libcxxabi/src/demangle/cp-to-llvm.sh.

libcxxabi/src/demangle/StringViewExtras.h

 #define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
 
 #include <string_view>
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
-  return !self.empty() && self.front() == C;
+inline bool starts_with(std::string_view self, char C) noexcept {
+  return !self.empty() && *self.begin() == C;
 }
 
-inline bool starts_with(std::string_view haystack, std::string_view needle) {
-  return haystack.substr(0, needle.size()) == needle;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.empty())
+    return true;
+  if (needle.size() > haystack.size())
+    return false;
+  haystack.remove_suffix(haystack.size() - needle.size());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END
 
 #endif

llvm/include/llvm/Demangle/StringViewExtras.h

 #define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
 
 #include <string_view>
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
-  return !self.empty() && self.front() == C;
+inline bool starts_with(std::string_view self, char C) noexcept {
+  return !self.empty() && *self.begin() == C;
 }
 
-inline bool starts_with(std::string_view haystack, std::string_view needle) {
-  return haystack.substr(0, needle.size()) == needle;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.empty())
+    return true;
+  if (needle.size() > haystack.size())
+    return false;
+  haystack.remove_suffix(haystack.size() - needle.size());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END
 
 #endif