Avoid C/C++ math function ambiguity for explicit module builds on Windows (#85609)

This tweaks the existing logic that disables the import of the C++
math functions to avoid ambiguity with the C math functions by
handling the case where the module directory can be null in explicit
module builds.

For #85606

Author: hjyamauchi

lib/ClangImporter/ImportDecl.cpp

@@ -3893,9 +3893,10 @@ namespace {
           if (auto file = sourceManager.getFileEntryRefForID(
                   sourceManager.getFileID(decl->getLocation()))) {
             auto filename = file->getName();
-            if ((file->getDir() == owningModule->Directory) &&
-                (filename.ends_with("cmath") || filename.ends_with("math.h") ||
-                 filename.ends_with("stdlib.h") || filename.ends_with("cstdlib"))) {
+            if (filename.ends_with("cmath") || filename.ends_with("math.h") ||
+                ((filename.ends_with("stdlib.h") || filename.ends_with("cstdlib")) &&
+                 decl->getDeclName().isIdentifier() &&
+                 (decl->getName() == "abs" || decl->getName() == "div"))) {
               return nullptr;
             }
           }

test/ScanDependencies/win-crt-explicit-cxx.swift

@@ -0,0 +1,54 @@
+// REQUIRES: OS=windows-msvc
+
+// Test building the CRT module with C++ interop enabled on Windows
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -scan-dependencies -module-name Test \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   -parse-stdlib -module-load-mode prefer-serialized \
+// RUN:   %t/Test.swift -o %t/deps.json -I %t 2>&1
+
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:SAL > %t/SAL.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:vcruntime > %t/vcruntime.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:SwiftShims > %t/SwiftShims.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json Swift > %t/Swift.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_Builtin_stddef > %t/_Builtin_stddef.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:corecrt > %t/corecrt.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:std_config > %t/std_config.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_float > %t/_float.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_fenv > %t/_fenv.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_stdlib > %t/_stdlib.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_malloc > %t/_malloc.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_Builtin_intrinsics > %t/_Builtin_intrinsics.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:ucrt > %t/ucrt.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:std > %t/std.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:_complex > %t/_complex.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:SwiftOverlayShims > %t/SwiftOverlayShims.cmd
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json CRT > %t/CRT.cmd
+
+// Remove the candidate module file flag to force building from the swiftinterface file.
+// RUN: cat %t/CRT.cmd | sed '/candidate-module-file/d; /CRT.swiftmodule.*swiftmodule/d' > %t/CRT.mod.cmd
+
+// RUN: %swift_frontend_plain @%t/SAL.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/vcruntime.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/SwiftShims.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/Swift.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_Builtin_stddef.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/corecrt.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/std_config.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_float.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_fenv.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_stdlib.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_malloc.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_Builtin_intrinsics.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/ucrt.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/std.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/_complex.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/SwiftOverlayShims.cmd 2>&1
+// RUN: %swift_frontend_plain @%t/CRT.mod.cmd 2>&1
+
+//--- Test.swift
+import CRT
+