When compiling with `-fembed-bitcode-marker`, Clang inserts a
placeholder
for the bitcode. This placeholder is a `[0 x i8]` array, which we cannot
represent in SPIRV.
For AMD flavored SPIRV, we extend the `llvm.embedded.module` global to a
`zeroinitializer [1 x i8]` array.
To achieve this, this patch adds a new pass, `SPIRVPrepareGlobals`, that
we can use to write global variable's _non-trivial-to-lower-IR_ ->
_trivial-to-lower-IR_ mappings.
This is a second attempt at
https://github.com/llvm/llvm-project/pull/162082, but cleaner.
In the translator something similar is done for every 0-element array
since https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2743 .
But I don't think we want to do this mapping for all cases.
