Fix -warn-stack-size command line option for LLVM 12
Fix -warn-stack-size command line option for LLVM 12

The -warn-stack-size command line option won't be deprecated until the
LLVM 13 release.

This change also produces a compile-time error when LLVM 13+ is used so
we won't fail to notice that the the stack size checking functionality
has to be implemented using the per-function "warn-stack-size" attribute
introduced by https://reviews.llvm.org/D104342. Note that development
builds of LLVM use LLVM_MAJOR_VERSION=9999.

Bug: b/191193823
Change-Id: I044d576a6dcd73354710783a7f7e042cb43254c5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55188
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>

google/swiftshader,google/swiftshader,google/swiftshader