[clang][ConstantEmitter] have tryEmitPrivate[ForVarInit] try ConstExprEmitter fast-path first

As suggested by @efriedma in:
https://reviews.llvm.org/D76096#4370369

This should speed up evaluating whether an expression is constant or
not, but due to the complexity of these two different implementations,
we may start getting different answers for edge cases for which we do
not yet have test cases in-tree (or perhaps even performance regressions
for some cases). As such, contributors have carte blanche to revert if
necessary.

For additional historical context about ExprConstant vs CGExprConstant,
here's snippets from a private conversation on discord:

  ndesaulniers:
  why do we have clang/lib/AST/ExprConstant.cpp and
  clang/lib/CodeGen/CGExprConstant.cpp? Does clang constant fold during
  ast walking/creation AND during LLVM codegen?
  efriedma:
  originally, clang needed to handle two things: integer constant
  expressions (the "5" in "int x[5];"), and constant global initializers
  (the "5" in "int x = 5;").  pre-C++11, the two could be handled mostly
  separately; so we had the code for integer constants in AST/, and the
  code for globals in CodeGen/.  C++11 constexpr sort of destroyed that
  separation, though. so now we do both kinds of constant evaluation on
  the AST, then CGExprConstant translates the result of that evaluation
  to LLVM IR.  but we kept around some bits of the old cgexprconstant to
  avoid performance/memory usage regressions on large arrays.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D151587