[clang] [HLSL] Make fast math the default for HLSL (PR #119820)

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

This looks correct, but how much work is it to update all of these tests to have fast math enabled rather than explicitly disabling it? I worry that (1) folks looking at these tests in the future will think that disabling fast math is a meaningful part of the test and (2) people will crib these run lines and add more tests that build with non-default flags for no particular reason

https://github.com/llvm/llvm-project/pull/119820