[libc][bazel] Add a flag to configure LIBC_NAMESPACE (#68093)

Author: gchatelet

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -11,7 +11,7 @@ load(
 )
 load(":platforms.bzl", "PLATFORM_CPU_ARM64", "PLATFORM_CPU_X86_64")
 load("@bazel_skylib//lib:selects.bzl", "selects")
-load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag")
 load("//:vars.bzl", "LLVM_VERSION_MAJOR", "LLVM_VERSION_MINOR", "LLVM_VERSION_PATCH")
 
 package(
@@ -35,8 +35,6 @@ MEMORY_COPTS = [
     # "LIBC_COPT_MEMCPY_X86_USE_SOFTWARE_PREFETCHING",
 ]
 
-llvm_libc_namespace = "__llvm_libc_{}_{}_{}_git".format(LLVM_VERSION_MAJOR, LLVM_VERSION_MINOR, LLVM_VERSION_PATCH)
-
 # A flag to pick which `mpfr` to use for math tests.
 # Usage: `--@llvm-project//libc:mpfr=<disable|external|system>`.
 # Flag documentation: https://bazel.build/extending/config
@@ -65,14 +63,34 @@ config_setting(
     flag_values = {":mpfr": "system"},
 )
 
+default_libc_namespace = "__llvm_libc_{}_{}_{}_git".format(LLVM_VERSION_MAJOR, LLVM_VERSION_MINOR, LLVM_VERSION_PATCH)
+
+release_libc_namespace = "__llvm_libc"
+
+# When set, The ':libc_root' target below will define 'LIBC_NAMESPACE' to
+# 'release_libc_namespace' instead of 'default_libc_namespace'.
+# Usage: `--@llvm-project//libc:release`.
+bool_flag(
+    name = "release",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "use_release_namespace",
+    flag_values = {":release": "true"},
+)
+
 # This empty root library helps us add an include path to this directory
 # using the 'includes' attribute. The strings listed in the includes attribute
 # are relative paths wrt this library but are inherited by the dependents
 # appropriately. Hence, using this as a root dependency avoids adding include
 # paths of the kind "../../" to other libc targets.
 cc_library(
     name = "libc_root",
-    defines = ["LIBC_NAMESPACE=" + llvm_libc_namespace],
+    defines = select({
+        ":use_release_namespace": ["LIBC_NAMESPACE=" + release_libc_namespace],
+        "//conditions:default": ["LIBC_NAMESPACE=" + default_libc_namespace],
+    }),
     includes = ["."],
 )