All deserialized VarDecl initializers are EvaluatedStmt, but not all
EvaluatedStmt initializers are from a PCH. Calling
`VarDecl::hasInitWithSideEffects` can trigger constant evaluation, but
it's hard to know ahead of time whether that will trigger
deserialization - even if the initializer is fully deserialized, it may
contain a call to a constructor whose body is not deserialized. By
caching the result of `VarDecl::hasInitWithSideEffects` and populating
that cache during deserialization we can guarantee that calling it won't
trigger deserialization regardless of the state of the initializer.
This also reduces memory usage by removing the `InitSideEffectVars` set
in `ASTReader`.

rdar://154717930
(cherry picked from commit 2910c24)

EvaluateAsInitializer does not support evaluating values with dependent
types. This was previously guarded with a check for the initializer
expression, but it is possible for the VarDecl to have a dependent type
without the initializer having a dependent type, when the initializer is
a specialized template type and the VarDecl has the unspecialized type.
This adds a guard checking for dependence in the VarDecl type as well.
This fixes the issue raised by Google in
llvm#145447

(cherry picked from commit c885005)