Optionally demote fatal errors to non-fatal errors.

This behavior is enabled when the new CXTranslationUnit_KeepGoing
option is passed to clang_parseTranslationUnit{,2}. It is geared
towards use by IDEs and similar consumers of the clang-c API where
fatal errors may arise when parsing incomplete code mid-edit, or
when include paths are not properly configured yet. In such situations
one still wants to get as much information as possible about a TU.
Previously, the semantic analysis would not instantiate templates
or report additional fatal errors after the first fatal error was
encountered.

Fixes PR24268.

Patch by Milian Wolff.

llvm-svn: 262318