[libclang] Record parsing invocation to a temporary file when requested

by client

This patch extends libclang by allowing it to record parsing operations to a
temporary JSON file. The file is deleted after parsing succeeds. When a crash
happens during parsing, the file is preserved and the client will be able to use
it to generate a reproducer for the crash.

These files are not emitted by default, and the client has to specify the
invocation emission path first.

rdar://35322543

Differential Revision: https://reviews.llvm.org/D40527

llvm-svn: 319702