Handle non-inlined clang::Type::getAs specializations in extract_symbols.py

The existing logic was to discard any symbols representing function template
instantiations, as the definitions were assumed to be inline. But there are
three explicit specializations of clang::Type::getAs that are only defined in
Clang's lib/AST/Type.cpp, and at least the plugin used by the LibreOffice build
(https://wiki.documentfoundation.org/Development/Clang_plugins) uses those
functions.

Differential Revision: https://reviews.llvm.org/D26455

llvm-svn: 286841