Clean up expr substitution and cloning.

Before: The pre- and postconditions of expr substitution and cloning,
in particular, their effect on the isAnalyzed_ flag were unclear and
sometimes inconsistent e.g., some literal exprs set isAnalyzed_ to
true in their c'tor. As a result, several places required ad-hoc
solutions like Expr.unsetIsAnalyzed() and Expr.reanalyze().

This patch cleans up expr substitution and cloning, summarized as follows:

Expr analysis:
All exprs start our with isAnalyzed_ = false. The flag it set to true
iff analyze() has been called on the expr.

Expr.clone():
Creates a deep copy of an expr including all its analysis state.

Expr.equals():
Comparison of expr trees ignores implicit casts. This simplifies expr
substitution because un/analyzed exprs can be easily compared/substituted.

ExprSubstitutionMap:
When adding a mapping, the rhs expr must be analyzed to allow
substitution across query blocks. There is no requirement on the lhs expr.

Expr substitution:
Substitution returns an analyzed clone of the original expr with exprs
substituted. While performing the substitution, implicit casts and analysis
state are removed such that the returned result has minimal implicit casts
and types.
There are two versions of substitute functions: One that throws exceptions
one that does not, because the caller may have different expectations on
whether a substitution must succeed or not.

Numeric literals:
This patch combines IntLiteral and DecimalLiteral into a NumericLiteral.
Its main benefit is that analyze() always produces the same type, even if
the literal was implicitly cast and/or isAnalyzed was unset because of
expr substitution. This was not the case before because an implicit cast
could permanently turn an IntLiteral into a DecimalLiteral.

There is no more need for unsetIsAnalyzed() or reanalyze().

Change-Id: I646110e3714cff8ae8d5a378c25a107dd43334b6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3228
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins