[clang] [HLSL] Don't invoke `dxv` from `clang-dxc` for text output (PR #135876)

[Justin Bogner via cfe-commits]

================
@@ -309,6 +309,9 @@ HLSLToolChain::TranslateArgs(const DerivedArgList &Args, StringRef BoundArch,
 }
 
 bool HLSLToolChain::requiresValidation(DerivedArgList &Args) const {
+  if (!Args.hasArg(options::OPT_dxc_Fo))
+    return false;
----------------
bogner wrote:

I'm not sure I understand what you mean. The validator runs on the binary dxil container - if we're just emitting textual dxil then there's nothing we can run the validator on. Currently what happens in that case is that we attempt to run the validator but it falls over because the input isn't something it understands.

that is,
```
$ ./bin/clang-dxc.exe -T cs_6_2 -Fc x.s e:/tmp/x.hlsl
shader: expected top-level entity
source_filename = "E:/tmp/x.hlsl"
^

clang-dxc: error: dxv command failed with exit code 1 (use -v to see invocation)
```

or if we just output to stdout:
```
$ ./bin/clang-dxc.exe -T cs_6_2 e:/tmp/x.hlsl  >/dev/null
...
The system cannot find the file specified. -
clang-dxc: error: dxv command failed with exit code 1 (use -v to see invocation)
```

https://github.com/llvm/llvm-project/pull/135876