[libc] default enable -ftrivial-auto-var-init=pattern (llvm#78776)

Usage of uninitialized memory is a top memory safety issue in C++ codebases.
Help mitigate this somewhat by default initialize stack allocations to a
pattern (0xAA repeating).

Clang has received optimizations to sink these into control flow paths that
access such values to minimize the overhead of these added initializations.

If there's a measurable slowdown, we can add
-ftrivial-auto-var-init-max-size=<N> for some value N bytes if we have any
large stack allocations, or add attribute uninitialized to any variable
declarations.

Unsupported until GCC 12.1 / Clang 8.

Increases file size of libc.a from a full build by +8.79Ki (+0.2%).

libc/cmake/modules/CheckCompilerFeatures.cmake

@@ -57,3 +57,8 @@ foreach(feature IN LISTS ALL_COMPILER_FEATURES)
 endforeach()
 
 message(STATUS "Compiler features available: ${AVAILABLE_COMPILER_FEATURES}")
+
+### Compiler Feature Detection ###
+
+# clang-8+, gcc-12+
+check_cxx_compiler_flag("-ftrivial-auto-var-init=pattern" LIBC_CC_SUPPORTS_PATTERN_INIT)

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -41,6 +41,9 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fno-unwind-tables")
     list(APPEND compile_options "-fno-asynchronous-unwind-tables")
     list(APPEND compile_options "-fno-rtti")
+    if (LIBC_CC_SUPPORTS_PATTERN_INIT)
+      list(APPEND compile_options "-ftrivial-auto-var-init=pattern")
+    endif()
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
     # -DLIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.

utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl

@@ -81,7 +81,12 @@ def libc_function(
     # We use the explicit equals pattern here because append and += mutate the
     # original list, where this creates a new list and stores it in deps.
     copts = copts or []
-    copts = copts + ["-O3", "-fno-builtin", "-fno-lax-vector-conversions"]
+    copts = copts + [
+      "-O3",
+      "-fno-builtin",
+      "-fno-lax-vector-conversions",
+      "-ftrivial-auto-var-init=pattern"
+    ]
 
     # We compile the code twice, the first target is suffixed with ".__internal__" and contains the
     # C++ functions in the "LIBC_NAMESPACE" namespace. This allows us to test the function in the