[static-stdlib] Explicit autolink for RegexParser in StringProcessing (#60319)
_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.