interp: take care of constant globals

Constant globals can't have been modified, even if a pointer is passed
externally. Therefore, don't treat it as such in hasExternalStore.

In addition, it doesn't make sense to update values of constant globals
after the interp pass is finished. So don't do this.

TODO: track whether objects are actually modified and only update the
globals if this is the case.

interp/testdata/basic.ll

@@ -6,6 +6,7 @@ target triple = "x86_64--linux"
 @main.nonConst2 = global i64 0
 @main.someArray = global [8 x {i16, i32}] zeroinitializer
 @main.exportedValue = global [1 x i16*] [i16* @main.exposedValue1]
+@main.exportedConst = constant i64 42
 @main.exposedValue1 = global i16 0
 @main.exposedValue2 = global i16 0
 @main.insertedValue = global {i8, i32, {float, {i64, i16}}} zeroinitializer
@@ -62,6 +63,11 @@ entry:
   call void @modifyExternal(i32* bitcast ([1 x i16*]* @main.exportedValue to i32*))
   store i16 5, i16* @main.exposedValue1
 
+  ; Test that marking a constant as external still allows loading from it.
+  call void @readExternal(i32* bitcast (i64* @main.exportedConst to i32*))
+  %constLoad = load i64, i64 * @main.exportedConst
+  call void @runtime.printint64(i64 %constLoad)
+
   ; Test that this even propagates through functions.
   call void @modifyExternal(i32* bitcast (void ()* @willModifyGlobal to i32*))
   store i16 7, i16* @main.exposedValue2
@@ -96,6 +102,8 @@ declare i64 @someValue()
 
 declare void @modifyExternal(i32*)
 
+declare void @readExternal(i32*)
+
 ; This function will modify an external value. By passing this function as a
 ; function pointer to an external function, @main.exposedValue2 should be
 ; marked as external.