[clang] Register all LLVM targets in AllClangUnitTest main (llvm#144428)

Addresses feedback in
llvm#134196 (comment)

Makes the tests less sensitive to target registration from unrelated
test fixtures by registering everything up front.

Author: rnk

clang/unittests/AllClangUnitTests.cpp

@@ -0,0 +1,28 @@
+//===- clang/unittests/AllClangUnitTests.cpp ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/TargetSelect.h"
+
+namespace {
+struct RegisterAllLLVMTargets {
+  RegisterAllLLVMTargets();
+} gv;
+} // namespace
+
+// This dynamic initializer initializes all layers (TargetInfo, MC, CodeGen,
+// AsmPrinter, etc) of all LLVM targets. This matches what cc1_main does on
+// startup, and prevents tests from initializing some of the Target layers,
+// which can interfere with tests that assume that lower target layers are
+// registered if the TargetInfo is registered.
+RegisterAllLLVMTargets::RegisterAllLLVMTargets() {
+  llvm::InitializeAllTargetInfos();
+  llvm::InitializeAllTargets();
+  llvm::InitializeAllTargetMCs();
+  llvm::InitializeAllAsmPrinters();
+  llvm::InitializeAllAsmParsers();
+}

clang/unittests/CMakeLists.txt

@@ -117,6 +117,7 @@ get_property(LINK_LIBS GLOBAL PROPERTY CLANG_UNITTEST_LINK_LIBS)
 get_property(LLVM_COMPONENTS GLOBAL PROPERTY CLANG_UNITTEST_LLVM_COMPONENTS)
 add_distinct_clang_unittest(AllClangUnitTests
   ${SRCS}
+  AllClangUnitTests.cpp
   CLANG_LIBS
   ${CLANG_LIBS}
   LINK_LIBS