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

[Justin Bogner via cfe-commits]

bogner wrote:

Note: To see the bitcode writer crash, apply this patch and set up https://github.com/llvm-beanz/HLSLTest - Basic/simple.test hits an assert:

```
Assertion failed: ((Val & ~(~0U >> (32-NumBits))) == 0 && "High bits set!"), function Emit, file BitstreamWriter.h, line 284.
...
#7 0x000000010615b9e4 llvm::BitstreamWriter::EmitVBR(unsigned int, unsigned int) (.cold.1) (/Users/bogner/code/llvm.org/build/bin/clang-20+0x1053e79e4)
#8 0x00000001040219c0 void llvm::BitstreamWriter::EmitAbbreviatedField<unsigned int>(llvm::BitCodeAbbrevOp const&, unsigned int) (/Users/bogner/code/llvm.org/build/bin/clang-20+0x1032ad9c0)
#9 0x00000001041b6678 void llvm::BitstreamWriter::EmitRecordWithAbbrevImpl<unsigned int>(unsigned int, llvm::ArrayRef<unsigned int>, llvm::StringRef, std::__1::optional<unsigned int>) (/Users/bogner/code/llvm.org/build/bin/clang-20+0x103442678)
#10 0x00000001041b32a8 llvm::dxil::DXILBitcodeWriter::writeInstruction(llvm::Instruction const&, unsigned int, llvm::SmallVectorImpl<unsigned int>&) (/Users/bogner/code/llvm.org/build/bin/clang-20+0x10343f2a8)
```

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