[LinkerWrapper] Fix errors not exiting inside of the LTO pipeline

The LTO pipeline handles its errors using the diagnostics handler
callback function. We were not checking the results of these errors and
not properly returning an error code in the linker wrapper when errors
occured inside of the LTO pipeline. This patch adds a simple boolean
flag to indicate if the LTO backend failed to any reason and quit.

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D129423