Instead of storing built-ins in a std::unordered_map, we now generate
a series of switch statements using the hash value of the look-up
string. This works similarly to earlier implementation of looking up
unmangled built-ins.

Those built-ins that need to be initialized at run-time are stored as
member variables of TSymbolTable.

This increases compiler init performance significantly, as well as
increasing compiler perf test scores around 1-2%. Binary size is
larger than before though.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: If1dcd36f0d2b30c2ed315cdcf6e831ae9fe70c94
Reviewed-on: https://chromium-review.googlesource.com/960031Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>