Add an internal-only helper StringAppendAndOverwrite() similar to

StringResizeAndOverwrite() but optimized for repeated appends, using
exponential growth to ensure amortized complexity of increasing
a string size by a small amount is O(1).

Use this function to replace STLStringResizeUninitializedAmortized()

PiperOrigin-RevId: 825100704
Change-Id: Ife64b13301bc4288d68154b7c496f57aaad4b6a4

absl/strings/BUILD.bazel

@@ -100,6 +100,7 @@ cc_library(
         "string_view.h",
     ],
     deps = [
+        ":append_and_overwrite",
         ":charset",
         ":internal",
         ":resize_and_overwrite",
@@ -170,6 +171,32 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "append_and_overwrite",
+    hdrs = ["internal/append_and_overwrite.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":resize_and_overwrite",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "append_and_overwrite_test",
+    srcs = ["internal/append_and_overwrite_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":append_and_overwrite",
+        "//absl/log:absl_check",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "match_test",
     size = "small",
@@ -625,6 +652,7 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":append_and_overwrite",
         ":cord_internal",
         ":cordz_info",
         ":cordz_update_scope",