[clang] Rework how and when APValues are dumped

[clang] Rework how and when APValues are dumped

Currently APValues are dumped as a single string. This becomes quickly
completely unreadable since APValue is a tree-like structure. Even a simple
example is not pretty:

  struct S { int arr[4]; float f; };
  constexpr S s = { .arr = {1,2}, .f = 3.1415f };
  // Struct  fields: Array: Int: 1, Int: 2, 2 x Int: 0, Float: 3.141500e+00

With this patch this becomes:

  -Struct
   |-field: Array size=4
   | |-elements: Int 1, Int 2
   | `-filler: 2 x Int 0
   `-field: Float 3.141500e+00

Additionally APValues are currently only dumped as part of visiting a
ConstantExpr. This patch also dump the value of the initializer of constexpr
variable declarations:

  constexpr int foo(int a, int b) { return a + b - 42; }
  constexpr int a = 1, b = 2;
  constexpr int c = foo(a, b) > 0 ? foo(a, b) : foo(b, a);
  // VarDecl 0x62100008aec8 <col:3, col:57> col:17 c 'const int' constexpr cinit
  // |-value: Int -39
  // `-ConditionalOperator 0x62100008b4d0 <col:21, col:57> 'int'
  // <snip>

Do the above by moving the dump functions to TextNodeDumper which already has
the machinery to display trees. The cases APValue::LValue, APValue::MemberPointer
and APValue::AddrLabelDiff are left as they were before (unimplemented).

We try to display multiple elements on the same line if they are considered to
be "simple". This is to avoid wasting large amounts of vertical space in an
example like:

  constexpr int arr[8] = {0,1,2,3,4,5,6,7};
  // VarDecl 0x62100008bb78 <col:3, col:42> col:17 arr 'int const[8]' constexpr cinit
  // |-value: Array size=8
  // | |-elements: Int 0, Int 1, Int 2, Int 3
  // | `-elements: Int 4, Int 5, Int 6, Int 7

Differential Revision: https://reviews.llvm.org/D83183

Reviewed By: aaron.ballman