[clang-proto-fuzzer] Allow user-specified compiler arguments.

Summary:
Arguments can be specified after -ignore_remaining_args=1 to modify
the compiler invocation.  For example, the following command-line
will fuzz LLVM with a custom optimization level and target triple:
  clang-proto-fuzzer CORPUS/ -ignore_remaining_args -O3 \
      -triple arm64-apple-ios9

Reviewers: vitalybuka, kcc

Reviewed By: vitalybuka

Subscribers: aemerson, cfe-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D36882

llvm-svn: 311185