Add a constexpr constructor for TVariable

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I9b7e01801caa7235ac5e2d4212ea92e38c1f774d
Reviewed-on: https://chromium-review.googlesource.com/908752
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/translator/ImmutableString.h b/src/compiler/translator/ImmutableString.h
index 2caef8d..e077377 100644
--- a/src/compiler/translator/ImmutableString.h
+++ b/src/compiler/translator/ImmutableString.h
@@ -54,7 +54,8 @@
     {
     }
 
-    ImmutableString(const ImmutableString &) = default;
+    constexpr ImmutableString(const ImmutableString &) = default;
+
     ImmutableString &operator=(const ImmutableString &) = default;
 
     constexpr const char *data() const { return mData ? mData : ""; }