[cxx-interop] Extend libstdc++ modulemap

The modulemap needs to include the headers even if they are not supposed to be used directly from Swift:

If a C++ header is being imported into Swift, and the header includes a stdlib header, clang needs to correctly deduce that the stdlib header is a part of the stdlib module, and not a part of the C++ module that we're importing.

stdlib/public/Cxx/std/libstdcxx.modulemap

@@ -25,7 +25,76 @@ module std {
   module compat {
     module cassert {
       header "cassert"
-      requires cplusplus
+      export *
+    }
+
+    module cerrno {
+      header "cerrno"
+      export *
+    }
+
+    module cctype {
+      header "cctype"
+      export *
+    }
+
+    module cfloat {
+      header "cfloat"
+      export *
+    }
+
+    module ciso646 {
+      header "ciso646"
+      export *
+    }
+
+    module climits {
+      header "climits"
+      export *
+    }
+
+    module cmath {
+      header "cmath"
+      export *
+    }
+
+    module cstdarg {
+      header "cstdarg"
+      export *
+    }
+
+    module cstddef {
+      header "cstddef"
+      export *
+    }
+
+    module cstdio {
+      header "cstdio"
+      export *
+    }
+
+    module cstdlib {
+      header "cstdlib"
+      export *
+    }
+
+    module cstring {
+      header "cstring"
+      export *
+    }
+
+    module ctime {
+      header "ctime"
+      export *
+    }
+
+    module cwchar {
+      header "cwchar"
+      export *
+    }
+
+    module cwctype {
+      header "cwctype"
       export *
     }
   }

test/Interop/Cxx/stdlib/libstdcxx-module-interface.swift

@@ -10,6 +10,8 @@
 // This test is specific to libstdc++ and only runs on platforms where libstdc++ is used.
 // REQUIRES: OS=linux-gnu
 
+// CHECK-STD: import std.compat
+
 // CHECK-STD: enum std {
 // CHECK-STRING:   struct {{__CxxTemplateInstNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE|__CxxTemplateInstSs}} {
 // CHECK-STRING:     typealias value_type = std.__CxxTemplateInstSt11char_traitsIcE.char_type