Need a class for basic mangled names since
with the addition of GLSL types the number
of basic types is > 52 (a-z, A-Z), so we need
more than one character to represent a type

Bug: angleproject:3719
Change-Id: I98beee9d42a016cb0c017f56ab82538c89212e33
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1742221
Commit-Queue: Clemen Deng <clemendeng@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>