Make TVariable type immutable

This enables using constexpr types for built-in variables and some of
the variables created in AST transformations.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ie85b3c9872a071a7c023ced013b14ad91cff7cee
Reviewed-on: https://chromium-review.googlesource.com/868134
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/RewriteDoWhile.cpp b/src/compiler/translator/RewriteDoWhile.cpp
index 80b2cee..aab608f 100644
--- a/src/compiler/translator/RewriteDoWhile.cpp
+++ b/src/compiler/translator/RewriteDoWhile.cpp
@@ -11,6 +11,7 @@
 
 #include "compiler/translator/IntermNode_util.h"
 #include "compiler/translator/IntermTraverse.h"
+#include "compiler/translator/StaticType.h"
 
 namespace sh
 {
@@ -71,7 +72,7 @@
             }
 
             // Found a loop to change.
-            TType boolType(EbtBool);
+            const TType *boolType = StaticType::Get<EbtBool, EbpUndefined, EvqTemporary, 1, 1>();
             TVariable *conditionVariable = CreateTempVariable(mSymbolTable, boolType);
 
             // bool temp = false;