Rework enable_strict_const_semantics to consider the hierarchy in aggregate

Rework enable_strict_const_semantics to consider the hierarchy in aggregate

Summary:
Enable_strict_const_semantics is now an int flag instead of a bool, but a bool is still accepted by .hhconfig
  - `1` and `true` are equivalent, and they cause class `B1` defined below to raise an error
  - `2` now raises an error for the definition of `B2`.

Adds an error when there is an inheritance conflict, which was previously missing in the case where the child overrides the constant.

Previously,

```
trait T1 {
  const int X = 4;
}
trait T2 {
  const int X = 5;
}
class A {
  use T1;
}
class B1 /* error here */ extends A {
  use T2;
}
```
```
T1
 \
  A   T2
   \  /
    B1
```
This is an error on B1, but if we  declared X locally, the error would disappear
```
// no errors
class B2 extends A {
  const int X = 6;
  use T2;
}
```
Both cases are fatals under HHVM flag.

Also, this check fires for all kinds of constants (class, type, ctx) because it does not exlude the backing TypeStructure constants, unlike the `check_const` call.

Facebook

~5k errors in www https://www.internalfb.com/intern/everpaste/?color=0&handle=GK10pBDe_jFp6hgQAOEB0BcVaMk6bjEQAAAz

Reviewed By: CatherineGasnier

Differential Revision: D35886138

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