[bazel] Turn on LLVM_ENABLE_PLUGINS on posix

Since #141650, `LLVM_ABI inline` has had different linkage regarding
to LLVM_ENABLE_PLUGINS.

This was introduced in 8830e38 (llvmorg-21-init-14426-g8830e3802a92)
but reverted in #143419 due to breakage of msvc build.

Author: chapuni

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -32,6 +32,7 @@ posix_defines = [
     "BACKTRACE_HEADER=<execinfo.h>",
     r'LTDL_SHLIB_EXT=\".so\"',
     r'LLVM_PLUGIN_EXT=\".so\"',
+    "LLVM_ENABLE_PLUGINS=1",
     "LLVM_ENABLE_THREADS=1",
     "HAVE_DEREGISTER_FRAME=1",
     "HAVE_LIBPTHREAD=1",

utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/llvm-config.h

@@ -124,6 +124,9 @@
 /* Define to 1 if you have the DIA SDK installed, and to 0 if you don't. */
 #define LLVM_ENABLE_DIA_SDK 0
 
+/* Define if plugins enabled */
+/* LLVM_ENABLE_PLUGINS defined in Bazel */
+
 /* Define if building LLVM with LLVM_ENABLE_TELEMETRY */
 #define LLVM_ENABLE_TELEMETRY 1