[HIP] Support flush denormals bitcode

Summary:
Use oclc_daz_opt_on.amdgcn.bc bitcode when option fcuda-flush-denormal-to-zero is enabled, otherwise use oclc_daz_opt_off.amdgcn.bc bitcode. Added lit tests to verify that the correct bitcode is linked when -fcuda-flush-denormal-to-zero option is enabled or disabled.

Reviewers: yaxunl, scchan, b-sumner

Reviewed By: yaxunl, scchan, b-sumner

Subscribers: cfe-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D48493

llvm-svn: 335765