[C++20][Modules] Allow import for a header unit after #pragma (#111662)

Summary:
`#pragma` and headers that finish with them shouldn't prevent `import
"header_unit.h"` syntax.

Test Plan: check-clang

Author: dmpolukhin

clang/lib/Lex/Preprocessor.cpp

@@ -902,6 +902,10 @@ void Preprocessor::Lex(Token &Result) {
     case tok::r_brace:
       StdCXXImportSeqState.handleCloseBrace();
       break;
+#define PRAGMA_ANNOTATION(X) case tok::annot_##X:
+// For `#pragma ...` mimic ';'.
+#include "clang/Basic/TokenKinds.def"
+#undef PRAGMA_ANNOTATION
     // This token is injected to represent the translation of '#include "a.h"'
     // into "import a.h;". Mimic the notional ';'.
     case tok::annot_module_include:

clang/test/Headers/import_header_unit_after_pragma.cpp

@@ -0,0 +1,18 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -std=c++20 -emit-header-unit -xc++-user-header bz0.h
+// RUN: %clang_cc1 -verify -std=c++20 -emit-header-unit -xc++-user-header -fmodule-file=bz0.pcm bz.cpp
+
+//--- compare
+#pragma GCC visibility push(default)
+#pragma GCC visibility pop
+
+//--- bz0.h
+#include "compare"
+// expected-no-diagnostics
+
+//--- bz.cpp
+#include "compare"
+
+import "bz0.h"; // expected-warning {{the implementation of header units is in an experimental phase}}