Switch to BumpPtrAllocator for C-string storage (#6550)

This keeps the allocations cheap and simplifies the code. It was
inspired by the need to expand param files, but
no functionality changed yet.

common/string_helpers.cpp

@@ -18,6 +18,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Allocator.h"
 #include "llvm/Support/ConvertUTF.h"
 
 namespace Carbon {
@@ -211,33 +212,19 @@ auto StringRefContainsPointer(llvm::StringRef ref, const char* ptr) -> bool {
 
 auto BuildCStrArgs(llvm::StringRef tool_path,
                    llvm::ArrayRef<llvm::StringRef> args,
-                   llvm::OwningArrayRef<char>& cstr_arg_storage)
+                   llvm::BumpPtrAllocator& alloc)
     -> llvm::SmallVector<const char*, 64> {
-  return BuildCStrArgs(tool_path, /*prefix_args=*/{}, args, cstr_arg_storage);
+  return BuildCStrArgs(tool_path, /*prefix_args=*/{}, args, alloc);
 }
 
 auto BuildCStrArgs(llvm::StringRef tool_path,
                    llvm::ArrayRef<std::string> prefix_args,
                    llvm::ArrayRef<llvm::StringRef> args,
-                   llvm::OwningArrayRef<char>& cstr_arg_storage)
+                   llvm::BumpPtrAllocator& alloc)
     -> llvm::SmallVector<const char*, 64> {
-  // Render the arguments into null-terminated C-strings. Command lines can get
-  // quite long in build systems so this tries to minimize the memory allocation
-  // overhead.
-
-  // Precompute the total C-string data size needed.
-  int total_size = tool_path.size() + 1;
-  for (llvm::StringRef arg : args) {
-    // Accumulate both the string size and a null terminator byte.
-    total_size += arg.size() + 1;
-  }
-
-  // Allocate one chunk of storage for the actual C-strings, and reserve a
-  // vector of pointers into the storage.
-  cstr_arg_storage = llvm::OwningArrayRef<char>(total_size);
   ssize_t i = 0;
   auto make_cstr = [&](llvm::StringRef arg) {
-    char* cstr = &cstr_arg_storage[i];
+    char* cstr = alloc.Allocate<char>(arg.size() + 1);
     memcpy(cstr, arg.data(), arg.size());
     cstr[arg.size()] = '\0';
     i += arg.size() + 1;