[C++20] [Modules] Introduce reduced BMI (#75894)

Close https://github.com/llvm/llvm-project/issues/71034

See

https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755

This patch introduces reduced BMI, which doesn't contain the definitions
of functions and variables if its definitions won't contribute to the
ABI.

Testing is a big part of the patch. We want to make sure the reduced BMI
contains the same behavior with the existing and relatively stable
fatBMI. This is pretty helpful for further reduction.

The user interfaces part it left to following patches to ease the
reviewing.

1 files changed, 6 insertions, 4 deletions

diff --git a/clang/unittests/Serialization/ForceCheckFileInputTest.cpp b/clang/unittests/Serialization/ForceCheckFileInputTest.cpp
index ed0daa4..ad8892b 100644
--- a/clang/unittests/Serialization/ForceCheckFileInputTest.cpp
+++ b/clang/unittests/Serialization/ForceCheckFileInputTest.cpp
@@ -69,9 +69,9 @@ export int aa = 43;
     CIOpts.Diags = Diags;
     CIOpts.VFS = llvm::vfs::createPhysicalFileSystem();
 
-    const char *Args[] = {
-        "clang++",       "-std=c++20", "--precompile", "-working-directory",
-        TestDir.c_str(), "a.cppm",     "-o",           BMIPath.c_str()};
+    const char *Args[] = {"clang++",       "-std=c++20",
+                          "--precompile",  "-working-directory",
+                          TestDir.c_str(), "a.cppm"};
     std::shared_ptr<CompilerInvocation> Invocation =
         createInvocation(Args, CIOpts);
     EXPECT_TRUE(Invocation);
@@ -88,6 +88,8 @@ export int aa = 43;
     Instance.setDiagnostics(Diags.get());
     Instance.setInvocation(Invocation);
 
+    Instance.getFrontendOpts().OutputFile = BMIPath;
+
     if (auto VFSWithRemapping = createVFSFromCompilerInvocation(
             Instance.getInvocation(), Instance.getDiagnostics(), CIOpts.VFS))
       CIOpts.VFS = VFSWithRemapping;
@@ -95,7 +97,7 @@ export int aa = 43;
 
     Instance.getHeaderSearchOpts().ValidateASTInputFilesContent = true;
 
-    GenerateModuleInterfaceAction Action;
+    GenerateReducedModuleInterfaceAction Action;
     EXPECT_TRUE(Instance.ExecuteAction(Action));
     EXPECT_FALSE(Diags->hasErrorOccurred());
   }