Directly use TCMalloc rather than the system malloc.

This improve the toolchain's performance by about 10%.

It will also allow us to leverage TCMalloc's extensions to do heap
profiling and get other information about how efficiently we're using
the heap.

Note that currently this causes all of our builds to produce a warning
due to an issue with `rules_python` and multiple modules registering
python toolchains:
bazelbuild/rules_python#1818

.bazelrc

@@ -35,6 +35,9 @@ build:clang-tidy --copt=-Wno-unknown-pragmas
 build --flag_alias=use_target_config_carbon_rules=//bazel/carbon_rules:use_target_config_carbon_rules
 build --use_target_config_carbon_rules
 
+# Use TCMalloc by default.
+build --custom_malloc=@tcmalloc//tcmalloc
+
 # Default to using a disk cache to minimize re-building LLVM and Clang which we
 # try to avoid updating too frequently to minimize rebuild cost. The location
 # here can be overridden in the user configuration where needed.

MODULE.bazel

@@ -43,6 +43,17 @@ archive_override(
     urls = ["https://sourceforge.net/projects/perfmon2/files/libpfm4/libpfm-{0}.tar.gz".format(libpfm_version)],
 )
 
+# The registry has a snapshot, but upstream is active and not regularly marking
+# releases. We start with the BCR snapshot to avoid a miss and then override it
+# with a more recent commit.
+bazel_dep(name = "tcmalloc", version = "0.0.0-20240411-5ed309d", dev_dependency = True)
+git_override(
+    module_name = "tcmalloc",
+    # HEAD as of 2024-07-14.
+    commit = "923df94c922e0cd2d0512c1662d374f63c2c0c96",
+    remote = "https://github.com/google/tcmalloc.git",
+)
+
 bazel_dep(name = "rules_bison", version = "0.2.2")
 bazel_dep(name = "rules_flex", version = "0.2.1")
 bazel_dep(name = "rules_m4", version = "0.2.3")

bazel/cc_toolchains/clang_cc_toolchain_config.bzl

@@ -1076,10 +1076,13 @@ def _impl(ctx):
         target_system_name = identifier,
         target_cpu = ctx.attr.target_cpu,
 
+        # This is used to expose a "flag" that `config_setting` rules can use to
+        # determine if the compiler is Clang.
+        compiler = "clang",
+
         # These attributes aren't meaningful at all so just use placeholder
         # values.
         target_libc = "local",
-        compiler = "local",
         abi_version = "local",
         abi_libc_version = "local",