Use CodegenOpts::less when creating a TargetMachine for clang `-O1`

Summary:
Clang was initializing the TargetMachine with CodeGenOpt::Default
for O1. This change is aligning it on llc:

-O0: OptLevel = CodeGenOpt::None
-O1: OptLevel = CodeGenOpt::Less
-O2 -Os -Oz: OptLevel = CodeGenOpt::Default
-O3: OptLevel = CodeGenOpt::Aggressive

Reviewers: echristo, chandlerc

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28409

llvm-svn: 291276