The ShaderInterfaceVariableInfoMap information is specific to each
shader stage, since the locations are determined for each shader stage.
This change makes mVariableInfoMap a gl::ShaderMap<> to handle this,
which will make it easier to compile separable programs, determine the
variable locations, and transform the SPIR-V.

Bug: angleproject:3570
Change-Id: I28b71a37efe54bbcfe1dcd445fa03ee71e74f0a6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2062741
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>