[TargetMachine] error message unsupported code model

When the tiny code model is requested for a target machine that does not
support this, we get an error message (which is nice) but also this diagnostic
and request to submit a bug report:

    fatal error: error in backend: Target does not support the tiny CodeModel
    [Inferior 2 (process 31509) exited with code 0106]
    clang-9: error: clang frontend command failed with exit code 70 (use -v to see invocation)
    (gdb) clang version 9.0.0 (http://llvm.org/git/clang.git 29994b0c63a40f9c97c664170244a7bba5ecc15e) (http://llvm.org/git/llvm.git 95606fdf91c2d63a931e865f4b78b2e9828ddc74)
    Target: arm-arm-none-eabi
    Thread model: posix
    clang-9: note: diagnostic msg: PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
    clang-9: note: diagnostic msg:
    ********************
    PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
    Preprocessed source(s) and associated run script(s) are located at:
    clang-9: note: diagnostic msg: /tmp/tiny-dfe1a2.c
    clang-9: note: diagnostic msg: /tmp/tiny-dfe1a2.sh
    clang-9: note: diagnostic msg:

But this is not a bug, this is a feature. :-) Not only is this not a bug, this
is also pretty confusing. This patch causes just to print the fatal error and
not the diagnostic:

fatal error: error in backend: Target does not support the tiny CodeModel

Differential Revision: https://reviews.llvm.org/D62236

llvm-svn: 361370