[cxx-interop][SwiftCompilerSources] Add LLVM headers to the include paths

This allows the Swift compiler to find headers like `llvm/Support/Compiler.h` that are used by Swift headers.

Author: egorzhdan

SwiftCompilerSources/CMakeLists.txt

@@ -158,6 +158,10 @@ function(add_swift_compiler_modules_library name)
               "-emit-module-path" "${build_dir}/${module}.swiftmodule"
               "-parse-as-library" ${sources}
               "-wmo" ${swift_compile_options}
+              # LLVM modules and headers.
+              "-Xcc" "-I" "-Xcc" "${LLVM_MAIN_INCLUDE_DIR}"
+              # Generated LLVM headers.
+              "-Xcc" "-I" "-Xcc" "${LLVM_INCLUDE_DIR}"
               # Bridging modules and headers.
               "-Xcc" "-I" "-Xcc" "${SWIFT_SOURCE_DIR}/include"
               # Generated C headers.

SwiftCompilerSources/Package.swift

@@ -20,6 +20,8 @@ private extension Target {
       "-Xfrontend", "-enable-cxx-interop",
       // Bridging modules and headers
       "-Xcc", "-I", "-Xcc", "../include",
+      // LLVM modules and headers
+      "-Xcc", "-I", "-Xcc", "../../llvm-project/llvm/include",
       "-cross-module-optimization"
     ]),
   ]