Source-based code coverage incorrectly handles if constexpr block

[chfast]

When a if constexpr has a block {} and the condition is false, the source-based code coverage always marks the opening { as not-executed region (see line 4).

Tested with clang 14.

    1|       |template <int C>
    2|      1|int test() {
    3|      1|    if constexpr (C == 0)
  ------------------
  |  Branch (3:19): [Folded - Ignored]
  ------------------
    4|      0|    {
    5|      1|        return 1;
    6|      1|    }
    7|      1|    return 0;
    8|      1|}
    9|       |
   10|       |int main()
   11|      1|{
   12|      1|    return test<1>();
   13|      1|}

[chfast]

When viewing the AST of this program, the body of if constexpr is replaced with NullStmt. https://godbolt.org/z/x1zosvM43

|     |-IfStmt 0x557733b75ae0 <line:3:5, line:4:5> constexpr
|     | |-ConstantExpr 0x557733b75ab8 <line:3:19, col:24> 'bool'
|     | | |-value: Int 0
|     | | `-BinaryOperator 0x557733b75a98 <col:19, col:24> 'bool' '=='
|     | |   |-SubstNonTypeTemplateParmExpr 0x557733b75a78 <col:19> 'int'
|     | |   | |-NonTypeTemplateParmDecl 0x557733b752e8 <line:1:11, col:15> col:15 referenced 'int' depth 0 index 0 C
|     | |   | `-IntegerLiteral 0x557733b75a58 <line:3:19> 'int' 1
|     | |   `-IntegerLiteral 0x557733b75588 <col:24> 'int' 0
|     | `-NullStmt 0x557733b75ad8 <line:4:5>

[cor3ntin]

if consteval is also not handled correctly. see #57377

[hanickadot]

I have a fix in mentioned PR. There was incorrectly marked one character after if constexpr (condition) as uncovered. Now whole region of valid THEN or ELSE branch is marked as uncovered. Also there was a similar problem with if consteval where always first branch wasn't covered. This is first part of fix. In subsequent PR I will make the code as skipped so it won't glow red.