[modules] Fix merging of __va_list_tag's implicit special member functions.

We model predefined declarations as not being from AST files, but in most ways
they act as if they come from some implicit prebuilt module file imported
before all others. Therefore, if we see an update to the predefined 'struct
__va_list_tag' declaration (and we've already loaded any modules), it needs a
corresponding update record, even though it didn't technically come from an AST
file.

llvm-svn: 250134