Add TType::getBuiltInTypeNameString() that returns an unique char
pointer for each built-in type. They can be easily stored in an
std::set as required by EmulatePrecision, and the function can also
be used in TOutputGLSLBase.

BUG=angleproject:1437
TEST=angle_unittests

Change-Id: I9acfe1b149546dedeae058841cbabf0d9829cdc9
Reviewed-on: https://chromium-review.googlesource.com/358471Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>