Work around a problem with a static helper's formulation in release

builds introduced in r134972:

lib/CodeGen/CGExpr.cpp:1294:7: error: no matching function for call to 'EmitBitCastOfLValueToProperType'
lib/CodeGen/CGExpr.cpp:1278:1: note: candidate function not viable: no known conversion from 'CGBuilderTy' (aka 'IRBuilder<false>') to 'llvm::IRBuilder<> &' for 1st argument

This fixes the issue by passing CodeGenFunction on down, and using its
builder directly rather than passing just the builder down.

This may not be the best / cleanest fix, Chris please review. It at
least fixes builds.

llvm-svn: 134977