Fix an AccessedStorage assert for SIL global variables.
Allow round-tripping access to global variables. Previously,
AccessedStorage asserted that global variables were always associated
with a VarDecl. This was to ensure that AccessEnforcmentWMO always
recognized the global. Failing to recognize access to a global will
cause a miscompile.
SILGlobalVariable now has all the information needed by
SIL. Particularly, the 'isLet' flag. Simply replace VarDecl with
SILGlobalVariable in AccessEnforcmentWMO to eliminate the need for the
assert.
