Add bazel `iree_link_compiler_shared` flag. (#12508)

With prior changes, it is now possible to make Bazel mirror CMake with
respect to shared linking of the compiler. This flag does that. We
default it to false for safety (CMake defaults to true). I don't know
how much we will use this but having 1:1 correspondance of the build
system has some merit.

build_tools/bazel/iree.bazelrc

diff --git a/build_tools/bazel/iree.bazelrc b/build_tools/bazel/iree.bazelrc
index 3bf77da..827036a 100644
--- a/build_tools/bazel/iree.bazelrc
+++ b/build_tools/bazel/iree.bazelrc
@@ -5,14 +5,6 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 ###############################################################################
-# Short-hand config settings for IREE specific features.
-###############################################################################
-
-# Equivalent to CMake flag IREE_ENABLE_RUNTIME_TRACING.
-# Builds the runtime with tracing support enabled.
-build --flag_alias=iree_enable_runtime_tracing=@tracy_client//:runtime_enable
-
-###############################################################################
 # Common flags that apply to all configurations.
 # Use sparingly for things common to all compilers and platforms.
 ###############################################################################
@@ -45,6 +37,10 @@
 # Build settings flag aliases. See https://bazel.build/rules/config
 ###############################################################################
 build --flag_alias=iree_drivers=//runtime/src/iree/hal/drivers:enabled_drivers
+build --flag_alias=iree_link_compiler_shared=//compiler/src/iree/compiler/API2:link_shared
+# Equivalent to CMake flag IREE_ENABLE_RUNTIME_TRACING.
+# Builds the runtime with tracing support enabled.
+build --flag_alias=iree_enable_runtime_tracing=@tracy_client//:runtime_enable
 
 ###############################################################################
 # Options for "generic_gcc" builds

build_tools/bazel_to_cmake/bazel_to_cmake_converter.py

diff --git a/build_tools/bazel_to_cmake/bazel_to_cmake_converter.py b/build_tools/bazel_to_cmake/bazel_to_cmake_converter.py
index f07c60b..ba805a5 100644
--- a/build_tools/bazel_to_cmake/bazel_to_cmake_converter.py
+++ b/build_tools/bazel_to_cmake/bazel_to_cmake_converter.py
@@ -227,6 +227,9 @@
   def alias(self, *args, **kwargs):
     pass
 
+  def bool_flag(self, *args, **kwargs):
+    pass
+
   def load(self, *args, **kwargs):
     pass
 
@@ -307,9 +310,11 @@
   def platform_trampoline_deps(self, basename, path="base"):
     return [f"//{path}/internal:{basename}_internal"]
 
-  def select(self, d):
-    self._convert_unimplemented_function("select", str(d))
-    return d["//conditions:default"]
+  def select(self, selector):
+    default_value = selector.get("//conditions:default")
+    if default_value is None:
+      raise ValueError("bazel_to_cmake can only convert selects with a default")
+    return default_value
 
   def cc_library(self,
                  name,

compiler/src/iree/compiler/API2/BUILD

diff --git a/compiler/src/iree/compiler/API2/BUILD b/compiler/src/iree/compiler/API2/BUILD
index eabcec4..e44f131 100644
--- a/compiler/src/iree/compiler/API2/BUILD
+++ b/compiler/src/iree/compiler/API2/BUILD
@@ -4,6 +4,7 @@
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("//build_tools/bazel:build_defs.oss.bzl", "iree_compiler_cc_library")
 
 package(
@@ -49,12 +50,30 @@
     ],
 )
 
-# Bazel does not have a well-defined mechanism for managing
-# shared libraries in a cross-platform way. Therefore, we
-# only support static linking of tools. Separately, we do
-# build an actual shared library for platforms that support
-# it, but this is only done to support stub use-cases.
+iree_compiler_cc_library(
+    name = "SharedImpl",
+    srcs = [
+        "//lib:libIREECompiler.so",
+    ],
+    tags = ["skip-bazel_to_cmake"],
+)
+
+bool_flag(
+    name = "link_shared",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "link_shared_config",
+    flag_values = {
+        ":link_shared": "True",
+    },
+)
+
 alias(
     name = "Impl",
-    actual = ":StaticImpl",
+    actual = select({
+        ":link_shared_config": ":SharedImpl",
+        "//conditions:default": ":StaticImpl",
+    }),
 )