Mark StaticType related functions as constexpr

Fixes compilation on some versions of GCC and probably Clang.

Follow-up to http://crrev.com/c/786317

Bug: angleproject:1432
Change-Id: I3fc3ad0f65492f9543eb27fcdce6ca29a9ad06e5
Reviewed-on: https://chromium-review.googlesource.com/812220
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/compiler/translator/SymbolTable.cpp b/src/compiler/translator/SymbolTable.cpp
index adf1e4e..90d4c15 100644
--- a/src/compiler/translator/SymbolTable.cpp
+++ b/src/compiler/translator/SymbolTable.cpp
@@ -236,7 +236,7 @@
         pop();
 }
 
-bool IsGenType(const TType *type)
+constexpr bool IsGenType(const TType *type)
 {
     if (type)
     {
@@ -248,7 +248,7 @@
     return false;
 }
 
-bool IsVecType(const TType *type)
+constexpr bool IsVecType(const TType *type)
 {
     if (type)
     {