This patch is the first one in the series of using ShaderMap as
the container of the resources for each type of shader everywhere
in ANGLE.

This patch defines the new data structure ShaderMap and use it in
class Program and Compiler in ANGLE front-end.

The following work includes:
1. Use ShaderMap in D3D back-ends.
2. Use ShaderMap in Vulkan back-ends.

BUG=angleproject:2169

Change-Id: I1c284d95f5a071c45bb468901eabc15694fffe38
Reviewed-on: https://chromium-review.googlesource.com/1011722
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>