[Clang] Fix export declaration diagnostic message (#149059)

Change the error message from "export declaration can only be used
within a module purview" to "export declaration can only be used within
a module interface" to be technically accurate.

The previous message was misleading because export declarations are
actually within a module purview when used in module implementation
units, but they are only allowed in module interface units.

This addresses the issue pointed out in GitHub issue #149008 where
Bigcheese noted that the diagnostic wording was incorrect.

Fixes #149008

Author: duhbbx

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12368,7 +12368,7 @@ def err_export_using_internal : Error<
   "using declaration referring to %1 with %select{internal|module|unknown}0 "
   "linkage cannot be exported">;
 def err_export_not_in_module_interface : Error<
-  "export declaration can only be used within a module purview">;
+  "export declaration can only be used within a module interface">;
 def err_export_inline_not_defined : Error<
   "inline function not defined%select{| before the private module fragment}0">;
 def err_export_partition_impl : Error<

clang/test/CXX/drs/cwg8xx.cpp

@@ -9,10 +9,10 @@
 namespace cwg820 { // cwg820: 2.7
 export template <class T> struct B {};
 // cxx98-17-warning@-1 {{exported templates are unsupported}}
-// since-cxx20-error@-2 {{export declaration can only be used within a module purview}}
+// since-cxx20-error@-2 {{export declaration can only be used within a module interface}}
 export template<typename T> void f() {}
 // cxx98-17-warning@-1 {{exported templates are unsupported}}
-// since-cxx20-error@-2 {{export declaration can only be used within a module purview}}
+// since-cxx20-error@-2 {{export declaration can only be used within a module interface}}
 } // namespace cwg820
 
 namespace cwg873 { // cwg873: 3.0

clang/test/CXX/module/dcl.dcl/dcl.module/dcl.module.interface/p1.cppm

@@ -9,7 +9,7 @@
 
 //--- ExportDeclNotInModulePurview.cppm
 // expected-error@* {{missing 'export module' declaration in module interface unit}}
-export int b; // expected-error {{export declaration can only be used within a module purview}}
+export int b; // expected-error {{export declaration can only be used within a module interface}}
 
 //--- A.cppm
 // expected-no-diagnostics
@@ -18,7 +18,7 @@ export int a;
 
 //--- AddExport.cppm
 module A; // #module-decl
-export int b; // expected-error {{export declaration can only be used within a module purview}}
+export int b; // expected-error {{export declaration can only be used within a module interface}}
 // expected-note@#module-decl {{add 'export' here}}
 
 //--- AddExport2.cppm

clang/test/CXX/module/module.interface/p1.cpp

@@ -7,7 +7,7 @@
 
 //--- errors.cpp
 module;
-export int a; // expected-error {{export declaration can only be used within a module purview}}
+export int a; // expected-error {{export declaration can only be used within a module interface}}
 export module M;
 export int b; // #1
 namespace N {
@@ -37,8 +37,8 @@ namespace N {
 //--- impl.cpp
 module M; // #M
 
-export int b2; // expected-error {{export declaration can only be used within a module purview}}
+export int b2; // expected-error {{export declaration can only be used within a module interface}}
 namespace N {
-  export int c2; // expected-error {{export declaration can only be used within a module purview}}
+  export int c2; // expected-error {{export declaration can only be used within a module interface}}
 }
 // expected-note@#M 2+{{add 'export'}}

clang/test/Modules/cxx20-10-2-ex1.cpp

@@ -14,7 +14,7 @@ export int x;
 module;
 
 #include "std-10-2-ex1.h"
-// expected-error@std-10-2-ex1.h:* {{export declaration can only be used within a module purview}}
+// expected-error@std-10-2-ex1.h:* {{export declaration can only be used within a module interface}}
 
 export module M1;
 export namespace {} // expected-error {{anonymous namespaces cannot be exported}}

clang/test/Modules/cxx20-export-import.cpp

@@ -11,4 +11,4 @@
 export module dummy;
 
 //--- test.cpp
-export import dummy; // expected-error {{export declaration can only be used within a module purview}}
+export import dummy; // expected-error {{export declaration can only be used within a module interface}}

clang/test/Modules/cxx20-import-diagnostics-a.cpp

@@ -110,7 +110,7 @@ module;
 
 module AOK1;
 
-export import C; // expected-error {{export declaration can only be used within a module purview}}
+export import C; // expected-error {{export declaration can only be used within a module interface}}
 
 int theAnswer () { return 42; }
 

clang/test/Modules/export-in-non-modules.cpp

@@ -1,4 +1,4 @@
 // RUN: %clang_cc1 -std=c++20 %s -fsyntax-only -verify
-export struct Unit { // expected-error {{export declaration can only be used within a module purview}}
+export struct Unit { // expected-error {{export declaration can only be used within a module interface}}
   bool operator<(const Unit &);
 };