[C++20] [Modules] [Reduced BMI] Generate the function body from implicitly instantiated class and constant variables

After this patch, we will generate the function body from implicitly
instantiated class. This is important for consumers with same
template arguments. Otherwise the consumers won't see the function body.
Since the consumers won't instantiate the templates again if they find an
instantiation.

Also we will generate the variable definition if the variable is
non-inline but known as constant. Such variables may not affect the
ABI, but they may get involved into the compile time constant computation
in the consumer's code. So we have to generate such definitions.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -281,11 +281,18 @@ bool clang::CanElideDeclDef(const Decl *D) {
 
     if (FD->isDependentContext())
       return false;
+
+    if (FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
+      return false;
   }
 
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     if (!VD->getDeclContext()->getRedeclContext()->isFileContext() ||
-        VD->isInline() || VD->isConstexpr() || isa<ParmVarDecl>(VD))
+        VD->isInline() || VD->isConstexpr() || isa<ParmVarDecl>(VD) ||
+        // Constant initialized variable may not affect the ABI, but they
+        // may be used in constant evaluation in the frontend, so we have
+        // to remain them.
+        VD->hasConstantInitialization())
       return false;
 
     if (VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)

clang/test/Modules/reduced-bmi-generating-codes.cppm

@@ -0,0 +1,40 @@
+// Although the reduced BMI are not designed to be generated,
+// it is helpful for testing whether we've reduced the definitions.
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/a.cppm \
+// RUN:     -emit-reduced-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/b.cpp \
+// RUN:     -fmodule-file=a=%t/a.pcm -S -emit-llvm -o - \
+// RUN:     | FileCheck %t/b.cpp
+
+//--- a.cppm
+export module a;
+
+export template <class T>
+class A {
+public:
+    int member() {
+        return 43;
+    }
+};
+
+// Instantiate `A<int>::member()`.
+export int a_member = A<int>().member();
+
+export const int a = 43;
+
+//--- b.cpp
+import a;
+
+static_assert(a == 43);
+
+int b() {
+    A<int> a;
+    return a.member();
+}
+
+// CHECK: define{{.*}}@_ZNW1a1AIiE6memberEv