Store symbol names as a ImmutableString This will enable compile-time initialization of built-in symbols as well as reducing copying strings. Most of the code that deals with names is changed to use ImmutableString where it makes sense to avoid conversions. The lexer/parser now allocate const char pointers into pool memory instead of allocating TStrings. These are then converted to ImmutableString upon entering TParseContext. BUG=angleproject:2267 TEST=angle_unittests, angle_end2end_tests Change-Id: I244d6271ea1ecf7150d4f89dfa388a7745a1150c Reviewed-on: https://chromium-review.googlesource.com/881561 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>