[Driver] Avoid repeated hash lookups (NFC) (#111468)

Author: kazutakahirata

clang/lib/Driver/MultilibBuilder.cpp

@@ -74,13 +74,11 @@ bool MultilibBuilder::isValid() const {
   llvm::StringMap<int> FlagSet;
   for (unsigned I = 0, N = Flags.size(); I != N; ++I) {
     StringRef Flag(Flags[I]);
-    llvm::StringMap<int>::iterator SI = FlagSet.find(Flag.substr(1));
+    auto [SI, Inserted] = FlagSet.try_emplace(Flag.substr(1), I);
 
     assert(StringRef(Flag).front() == '-' || StringRef(Flag).front() == '!');
 
-    if (SI == FlagSet.end())
-      FlagSet[Flag.substr(1)] = I;
-    else if (Flags[I] != Flags[SI->getValue()])
+    if (!Inserted && Flags[I] != Flags[SI->getValue()])
       return false;
   }
   return true;