[ModuleCache] Prefer module cache passed on the command-line for checker

When setting up the ModuleInterfaceChecker, prefer using the module
cache path from command-line invocation `-module-cache-path` before
falling back to clang options.

Usually those two yield the same result, except for LLDB under direct
cc1 argument mode and explicit module build. Under such mode, the cc1
option for module cache path will be stripped since the output PCMs are
explicit passed as output. When LLDB attempted to do an implicit module
compilation for the swift interface, it will not be able to locate the
module cache path from cc1 arguments. On the other hand, the module
cache option has already be inherited by the sub-instance so it can just
directly be located there.

rdar://137610484

Author: cachemeifyoucan

lib/Frontend/Frontend.cpp

@@ -818,8 +818,11 @@ bool CompilerInstance::setUpModuleLoaders() {
   }
 
   // Configure ModuleInterfaceChecker for the ASTContext.
+  auto CacheFromInvocation = getInvocation().getClangModuleCachePath();
   auto const &Clang = clangImporter->getClangInstance();
-  std::string ModuleCachePath = getModuleCachePathFromClang(Clang);
+  std::string ModuleCachePath = CacheFromInvocation.empty()
+                                    ? getModuleCachePathFromClang(Clang)
+                                    : CacheFromInvocation.str();
   auto &FEOpts = Invocation.getFrontendOptions();
   ModuleInterfaceLoaderOptions LoaderOpts(FEOpts);
   Context->addModuleInterfaceChecker(

test/ModuleInterface/ModuleCache/module-cache-options.swift

@@ -0,0 +1,39 @@
+// UNSUPPORTED: OS=windows-msvc
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -parse-stdlib -I %t -emit-module-interface-path %t/LeafModule.swiftinterface -module-name LeafModule %t/leaf.swift -emit-module -o /dev/null
+// RUN: %target-swift-frontend -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -parse-stdlib -I %t -module-cache-path %t/swiftcache -emit-module-interface-path %t/OtherModule.swiftinterface -module-name OtherModule %t/other.swift -emit-module -o /dev/null
+// RUN: %target-swift-frontend -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -parse-stdlib -I %t -module-cache-path %t/swiftcache -Xcc -fmodules-cache-path=%t/clangcache -emit-module -o %t/TestModule.swiftmodule -module-name TestModule %t/main.swift
+
+// RUN: NUM_SWIFT_MODULES=$(find %t/swiftcache -type f -name '*.swiftmodule' | wc -l)
+// RUN: NUM_CLANG_MODULES=$(find %t/clangcache -type f -name '*.pcm' | wc -l)
+/// Two swift modules, Leaf and Other
+// RUN: if [ ! $NUM_SWIFT_MODULES -eq 2 ]; then echo "Should only be 2 Swift Modules, found $NUM_SWIFT_MODULES"; exit 1; fi
+/// Two clang modules, shim and A
+// RUN: if [ ! $NUM_CLANG_MODULES -eq 2 ]; then echo "Should only be 2 Clang Modules, found $NUM_CLANG_MODULES"; exit 1; fi
+
+//--- leaf.swift
+public func LeafFunc() {}
+
+//--- other.swift
+import LeafModule
+public func OtherFunc() {}
+
+//--- module.modulemap
+module A {
+  header "A.h"
+  export *
+}
+
+//--- A.h
+void a(void);
+
+//--- main.swift
+import OtherModule
+import A
+
+public func TestFunc() {
+    OtherFunc()
+    a()
+}