Add the "ForceSizeOpt" attribute.

Add the "ForceSizeOpt" attribute.

Patch by Quentin Colombet <qcolombet@apple.com>

Original description:
"""
The attached patch is the first step to have a better control on Oz related optimizations.
The Oz optimization level focuses on code size, thus I propose to add an attribute called ForceSizeOpt.
"""

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166422 91177308-0d34-0410-b5e6-96231b3b80d8