Revert r107690 (for PR7417) and add a testcase that it breaks. The approach of

that change turns out to not be reasonable: mutating the AST of a parsed
template during instantiation is not a sound thing to do, does not work across
chained PCH / modules builds, and is in any case a special-case workaround to a
more general problem that should be solved centrally.

llvm-svn: 249342