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>
diff --git a/src/compiler/translator/util.cpp b/src/compiler/translator/util.cpp
index bd39cd1..9b70217 100644
--- a/src/compiler/translator/util.cpp
+++ b/src/compiler/translator/util.cpp
@@ -473,12 +473,12 @@
     return arrayString.str();
 }
 
-TString GetTypeName(const TType &type, ShHashFunction64 hashFunction, NameMap *nameMap)
+ImmutableString GetTypeName(const TType &type, ShHashFunction64 hashFunction, NameMap *nameMap)
 {
     if (type.getBasicType() == EbtStruct)
         return HashName(type.getStruct(), hashFunction, nameMap);
     else
-        return type.getBuiltInTypeNameString();
+        return ImmutableString(type.getBuiltInTypeNameString());
 }
 
 bool IsVaryingOut(TQualifier qualifier)