AST: simplify debug flag computation

Use an algorithmic approach to string concatenation rather than building
the space delimited string manually. This allows us to extend the flags
embedding more easily in the future.

Author: compnerd

include/swift/AST/IRGenOptions.h

@@ -32,6 +32,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/VersionTuple.h"
 #include <optional>
+#include <sstream>
 #include <string>
 #include <tuple>
 #include <vector>
@@ -656,24 +657,21 @@ class IRGenOptions {
   std::string getDebugFlags(StringRef PrivateDiscriminator,
                             bool EnableCXXInterop,
                             bool EnableEmbeddedSwift) const {
-    std::string Flags = DebugFlags;
+    llvm::SmallVector<std::string, 4> Flags{DebugFlags};
+
     if (!PrivateDiscriminator.empty()) {
-      if (!Flags.empty())
-        Flags += " ";
-      Flags += ("-private-discriminator " + PrivateDiscriminator).str();
-    }
-    if (EnableCXXInterop) {
-      if (!Flags.empty())
-        Flags += " ";
-      Flags += "-enable-experimental-cxx-interop";
+      Flags.emplace_back("-private-discriminator");
+      Flags.emplace_back(PrivateDiscriminator);
     }
-    if (EnableEmbeddedSwift) {
-      if (!Flags.empty())
-        Flags += " ";
-      Flags += "-enable-embedded-swift";
-    }
-
-    return Flags;
+    if (EnableCXXInterop)
+      Flags.emplace_back("-enable-experimental-cxx-interop");
+    if (EnableEmbeddedSwift)
+      Flags.emplace_back("-enable-embedded-swift");
+
+    std::ostringstream buffer;
+    std::copy(std::begin(Flags), std::end(Flags),
+              std::ostream_iterator<std::string>(buffer, " "));
+    return buffer.str();
   }
 
   /// Return a hash code of any components from these options that should