Override the SPIRV-Headers path to use local copy
Override the SPIRV-Headers path to use local copy

SPIRV-Tools' BUILD.gn file uses the spirv_tools_spirv_headers_dir
variable to provide the path to the SPIRV-Headers project. This variable
is set by //build_overrides/spirv_tools.gni, which when building as part
of Chromium points to //third_party/vulkan-deps/spirv-headers/src. That
version can be out of sync with SwiftShader's internal copy of SPIRV-
Tools.

Instead we want it to build with our internal copy of SPIRV-Headers.
This change accomplishes that by overriding the path to be relative
to the SPIRV-Tools directory, instead of being determined by
spirv_tools_spirv_headers_dir.

This is a local workaround to unblock autorollers. We'll need a proper
fix in the upstream SPIRV-Tools repo which can then be imported.

Bug: b/158002593
Change-Id: I0a19539230bf470a9943478ac4f3a15061fe6070
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/65688
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,google/swiftshader,google/swiftshader