To support Program Pipeline Objects, we need to hold on to the shaders
in separable programs, rather than detaching them during
glDetachShader()/glCreateShaderProgramv(). This is necessary due to
requiring the shader information for validating shader interfaces,
varyings, etc.

Instead, a new ShaderMap of bools will be stored in
ProgramState::mAttachedShadersMarkedForDetach to track when a caller
attempts to detach a shader from a separable Program. Later, when a new
shader is attached, we will first validate that the old shader is marked
to be detached, and if so, release it then.

Bug: angleproject:3570
Change-Id: I63fac2e5914c1c1a73f0b37863bac0f185ecb44c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2084399Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>