[skslc] Support HLSL output

- skslc can now generate HLSL using spirv-cross if invokved with an output
file with the ".hlsl" extension
- The SkSL::SPIRVtoHLSL function used to be conditionally compiled based
on the SK_USE_DIRECT3D define, which gates D3D support for the entire
gpu backend. This function is now conditionally compiled using a new,
more narrowly scoped define, which is always enabled when building
skslc.

Bug: skia:12691
Test: Run:
Change-Id: I6967c26ab28954ec09e1c62513187c8986135290
      1. ./skslc ../../resources/sksl/spirv/Ossfuzz35916.sksl TEST.hlsl
      2. ./dxc -T ps_6_7 TEST.hlsl
      Ensure that 1 runs without any errors and 2 does not detect any
      issues in the hlsl.
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/482257
Auto-Submit: Arman Uguray <armansito@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed