[libclang] Allow crash recovery with LIBCLANG_NOTHREADS

Enabled crash recovery for some libclang operations on a calling thread even
when LIBCLANG_NOTHREAD is specified.

Previously it would only run under crash recovery if LIBCLANG_NOTHREAD is not
set. Moved handling of LIBCLANG_NOTHREAD env variable into RunSafely from its
call sites.

llvm-svn: 318142