[static-stdlib] Explicit autolink for RegexParser in StringProcessing

_RegexParser is used only in StringProcessing and it's imported with
@_implementationOnly.
Modules imported with @_implementationOnly are not autolinked, so build
system let the importer module how to link it. When building a shared
library, _RegexParser is already linked into StringProcessing. But when
building a static library, it's not. So we need to add autolink entry
explicitly.

Author: kateinoigakukun

stdlib/public/StringProcessing/CMakeLists.txt

@@ -14,6 +14,13 @@ set(swift_string_processing_link_libraries
   swiftCore
   swift_RegexParser)
 
+set(swift_string_processing_compile_flags)
+if(SWIFT_BUILD_STATIC_STDLIB)
+  # Explicitly autolink swift_RegexParser because it's imported with @_implementationOnly
+  list(APPEND swift_string_processing_compile_flags
+    "-Xfrontend" "-public-autolink-library" "-Xfrontend" "swift_RegexParser")
+endif()
+
 file(GLOB_RECURSE _STRING_PROCESSING_SOURCES
   "${EXPERIMENTAL_STRING_PROCESSING_SOURCE_DIR}/Sources/_StringProcessing/*.swift"
   "${EXPERIMENTAL_STRING_PROCESSING_SOURCE_DIR}/Sources/_CUnicode/*.h"
@@ -40,6 +47,7 @@ add_swift_target_library(swift_StringProcessing ${SWIFT_STDLIB_LIBRARY_BUILD_TYP
   C_COMPILE_FLAGS
     -Dswift_StringProcessing_EXPORTS
   SWIFT_COMPILE_FLAGS
+    ${swift_string_processing_compile_flags}
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"