Support C++20 Modules in clang-repl (#79261)

This comes from when I playing around clang-repl with moduels : )

I succeeded to import std with https://libcxx.llvm.org/Modules.html and
calling `std::printf` after this patch.

I want to put the documentation part to
https://clang.llvm.org/docs/StandardCPlusPlusModules.html in a separate
commit.

Author: ChuanqiXu9

clang/docs/ReleaseNotes.rst

@@ -191,6 +191,8 @@ Crash and bug fixes
 Improvements
 ^^^^^^^^^^^^
 
+- Support importing C++20 modules in clang-repl.
+
 Moved checkers
 ^^^^^^^^^^^^^^
 

clang/include/clang/Basic/LangOptions.def

@@ -485,7 +485,7 @@ VALUE_LANGOPT(FuchsiaAPILevel, 32, 0, "Fuchsia API level")
 // on large _BitInts.
 BENIGN_VALUE_LANGOPT(MaxBitIntWidth, 32, 128, "Maximum width of a _BitInt")
 
-LANGOPT(IncrementalExtensions, 1, 0, " True if we want to process statements"
+COMPATIBLE_LANGOPT(IncrementalExtensions, 1, 0, " True if we want to process statements"
         "on the global scope, ignore EOF token and continue later on (thus "
         "avoid tearing the Lexer and etc. down). Controlled by "
         "-fincremental-extensions.")

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3286,10 +3286,10 @@ DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
   if (auto *OID = dyn_cast<ObjCInterfaceDecl>(DC))
     return OID->getDefinition();
 
-  // We can see the TU here only if we have no Sema object. In that case,
-  // there's no TU scope to look in, so using the DC alone is sufficient.
+  // We can see the TU here only if we have no Sema object. It is possible
+  // we're in clang-repl so we still need to get the primary context.
   if (auto *TU = dyn_cast<TranslationUnitDecl>(DC))
-    return TU;
+    return TU->getPrimaryContext();
 
   return nullptr;
 }

clang/test/Interpreter/cxx20-modules.cppm

@@ -0,0 +1,31 @@
+// UNSUPPORTED: system-aix
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/mod.cppm --precompile \
+// RUN:     -o %t/mod.pcm
+// RUN: %clang %t/mod.pcm -c -o %t/mod.o
+// RUN: %clang -shared %t/mod.o -o %t/libmod.so
+//
+// RUN: cat %t/import.cpp | env LD_LIBRARY_PATH=%t:$LD_LIBRARY_PATH \
+// RUN:     clang-repl -Xcc=-std=c++20 -Xcc=-fmodule-file=M=%t/mod.pcm \
+// RUN:     | FileCheck %t/import.cpp
+
+//--- mod.cppm
+export module M;
+export const char* Hello() {
+    return "Hello Interpreter for Modules!";
+}
+
+//--- import.cpp
+
+%lib libmod.so
+
+import M;
+
+extern "C" int printf(const char *, ...);
+printf("%s\n", Hello());
+
+// CHECK: Hello Interpreter for Modules!