Generate Libclang invocation reproducers using a new -cc1gen-reproducer

Generate Libclang invocation reproducers using a new -cc1gen-reproducer
driver option

This commit is a follow up to the previous work that recorded Libclang invocations
into temporary files: r319702.

It adds a new -cc1 mode to clang: -cc1gen-reproducer. The goal of this mode is to generate
Clang reproducer files for Libclang tool invocation. The JSON format in the invocation
files is not really intended to be stable, so Libclang and Clang should be of the same version
when generating reproducers.
The new mode emits the information about the temporary files and Libclang-specific information
to stdout using JSON.

rdar://35322614

Differential Revision: https://reviews.llvm.org/D40983

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329442 91177308-0d34-0410-b5e6-96231b3b80d8