Fix include paths to support both vendored and system-installed glslang.
Remove usage of the private `StandAlone` bits.
Requires us to vendor a copy of `DefaultTBuiltInResource` (or provide our own
customized one) as glslang doesn't provide it in its public API.
Also removes unused C interface as it's not well encapsulated and depends on
`StandAlone`.
Fixes#56307.
