Introduce a custom filesystem library (#5888)

The standard filesystem API lacks significant functionality, ranging
from correct and secure creation of directories and files within them by
using `openat` and avoiding [TOCTOU] issues, to support for filesystem
locking.

[TOCTOU]: https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use

The LLVM filesystem library has more functionality, but uses an API that
is increasingly diverging from the standard, and also fails to defend
against TOCTOU.

This library is designed to carefully model the Unix or POSIX filesystem
concepts of `openat` to avoid TOCTOU. However, it also tries to limit
itself to an API subset that LLVM's filesystem library has also
implemneted and so we have a strong reason to expect to be possible to
port to Windows reasonably.

This PR included several benchmarks that show that this implementation
is also faster for the majority of operations than the C++ standard
library. The only places where there is a consistent regression is in
recursively creating directories, and this is directly connected to the
approach of using `openat` as the basis. Even there, while the wall time
regresses, the cycles and instructions are significantly improved.

There are a number of operations not yet included here, I've focused on
a core set of opening, closing, creating, and removing, and then adding
those that I saw the current toolchain code using actively. I'll plan to
expand the operations as needed going forward.

A follow-up PR that I'll finish polishing and send next ports
`//toolchain/install` to consistently use this library and
`std::filesystem::path` to both exercise the library and showcase its
use. I'll be working systematically across the toolchain to converge all
the code, extending this library as needed.

For reference, benchmark results on my macOS laptop:
https://gist.github.com/chandlerc/29d1f4d465a835b8be5174a48dad2e8f

Benchmark results on a Asahi Linux M1 Mac Mini:
https://gist.github.com/chandlerc/c42d43dd6b9b91746ab314b2afa152f7

Benchmark results on a Linux server with weirdly slow FS operations:
https://gist.github.com/chandlerc/48301a7383eb3972d53351b7e35e0561

---------

Co-authored-by: Dana Jansens <[email protected]>

Author: chandlerc

common/BUILD

@@ -214,6 +214,56 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "filesystem",
+    srcs = ["filesystem.cpp"],
+    hdrs = ["filesystem.h"],
+    deps = [
+        ":build_data",
+        ":check",
+        ":error",
+        ":ostream",
+        ":raw_string_ostream",
+        ":template_string",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
+cc_test(
+    name = "filesystem_test",
+    size = "small",
+    srcs = ["filesystem_test.cpp"],
+    deps = [
+        ":error_test_helpers",
+        ":filesystem",
+        "//testing/base:gtest_main",
+        "@googletest//:gtest",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
+cc_binary(
+    name = "filesystem_benchmark",
+    testonly = 1,
+    srcs = ["filesystem_benchmark.cpp"],
+    deps = [
+        ":check",
+        ":filesystem",
+        "//testing/base:benchmark_main",
+        "@abseil-cpp//absl/hash",
+        "@abseil-cpp//absl/random",
+        "@google_benchmark//:benchmark",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
+sh_test(
+    name = "filesystem_benchmark_test",
+    size = "small",
+    srcs = [":filesystem_benchmark"],
+    args = ["--benchmark_min_time=1x"],
+)
+
 cc_library(
     name = "find",
     hdrs = ["find.h"],