New function discovery and support for multiple entries.

Summary:
Modified function discovery process to tolerate more functions and
symbols coming from assembly. The processing order now matches
the memory order of the functions (input symbol table is unsorted).

Added basic support for functions with multiple entries. When
a function references its internal address other than with
a branch instruction, that address could potentially escape.
We mark such addresses as entry points and make sure they
are treated as roots by unreachable code elimination.

Without relocations we have to mark multiple-entry functions
as non-simple.

(cherry picked from FBD3950243)

bolt/BinaryContext.cpp

@@ -57,6 +57,17 @@ MCSymbol *BinaryContext::getOrCreateGlobalSymbol(uint64_t Address,
   return Symbol;
 }
 
+MCSymbol *BinaryContext::getGlobalSymbolAtAddress(uint64_t Address) const {
+  auto NI = GlobalAddresses.find(Address);
+  if (NI == GlobalAddresses.end())
+    return nullptr;
+
+  auto *Symbol = Ctx->lookupSymbol(NI->second);
+  assert(Symbol && "symbol cannot be NULL at this point");
+
+  return Symbol;
+}
+
 void BinaryContext::printGlobalSymbols(raw_ostream& OS) const {
   for (auto &entry : GlobalSymbols) {
     OS << "(" << entry.first << " -> " << entry.second << ")\n";