Swift: Replace hyphens with underscores in default module name

GetSwiftModuleName() returns the target name verbatim when
Swift_MODULE_NAME is not set, but hyphens are not valid in Swift
module identifiers.  Replace them with underscores so that targets
like swift-syntax get module name swift_syntax automatically.

Issue: #27748

Help/prop_tgt/Swift_MODULE_NAME.rst

@@ -4,4 +4,5 @@ Swift_MODULE_NAME
 .. versionadded:: 3.15
 
 This property specifies the name of the Swift module.  It is defaulted to the
-name of the target.
+name of the target, with hyphens replaced by underscores (hyphens are not valid
+in Swift module identifiers).

Help/release/dev/swift-ninja-separate-swiftmodule-edge.rst

@@ -6,3 +6,6 @@ swift-ninja-separate-swiftmodule-edge
   the :variable:`CMAKE_Swift_SEPARATE_MODULE_EMISSION` variable, to control
   whether importable Swift targets emit ``.swiftmodule`` from a dedicated
   build edge.  Policy :policy:`CMP0215` enables this by default.
+
+* The default :prop_tgt:`Swift_MODULE_NAME` now replaces hyphens with
+  underscores, since hyphens are not valid in Swift module identifiers.

Source/cmGeneratorTarget.cxx

@@ -6173,7 +6173,13 @@ std::string cmGeneratorTarget::BuildDatabasePath(
 
 std::string cmGeneratorTarget::GetSwiftModuleName() const
 {
-  return this->GetPropertyOrDefault("Swift_MODULE_NAME", this->GetName());
+  if (cmValue name = this->GetProperty("Swift_MODULE_NAME")) {
+    return *name;
+  }
+  // Hyphens are not valid in Swift module identifiers.
+  std::string name = this->GetName();
+  std::replace(name.begin(), name.end(), '-', '_');
+  return name;
 }
 
 std::string cmGeneratorTarget::GetSwiftModuleFileName() const

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -171,6 +171,13 @@ if(RunCMake_GENERATOR MATCHES "Ninja")
     endif()
   endblock()
 
+  block()
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/SwiftModuleNameHyphen-build)
+    run_cmake(SwiftModuleNameHyphen)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    run_cmake_command(SwiftModuleNameHyphen-build ${CMAKE_COMMAND} --build . -- -vn)
+  endblock()
+
   block()
     set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/SwiftLibraryModuleCommand-build)
     run_cmake(SwiftLibraryModuleCommand)

Tests/RunCMake/Swift/SwiftModuleNameHyphen-build-check.cmake

@@ -0,0 +1,9 @@
+# The module name should have underscores, not hyphens.
+if(NOT actual_stdout MATCHES "-module-name L_hyphen")
+  string(APPEND RunCMake_TEST_FAILED
+    "Expected '-module-name L_hyphen', hyphens should be replaced with underscores.\n")
+endif()
+if(actual_stdout MATCHES "-module-name L-hyphen")
+  string(APPEND RunCMake_TEST_FAILED
+    "Found '-module-name L-hyphen', hyphens are not valid in Swift module names.\n")
+endif()

Tests/RunCMake/Swift/SwiftModuleNameHyphen.cmake

@@ -0,0 +1,11 @@
+cmake_policy(SET CMP0157 NEW)
+
+if(NOT CMAKE_GENERATOR MATCHES "Ninja")
+  message(SEND_ERROR "this test must use a Ninja generator, found ${CMAKE_GENERATOR}")
+endif()
+
+enable_language(Swift)
+
+# A target with hyphens in the name.  Swift module names cannot contain
+# hyphens, so CMake should replace them with underscores.
+add_library(L-hyphen STATIC L.swift)