[5.3][SourceKit/CodeFormat] Fix var/let declaration continuation lines appearing over-indented.

[nathawes]

Cherry-pick of #31941 for 5.3

• Explanation:
  The indentation of pattern binding decls was changed in 5.3 to improve their readability when they include multiple entries by column-aligning the entries and indenting their continuation lines relative to that column too. This change made pattern binding decls with a single entry that continued onto a second line seem over-indented, though, both relative to their surrounding code and to how they were treated in the 5.2 release, and single-entry pattern binding decls are used far more commonly than multiple-entry ones. This change fixes the over-indentation issue by not column-aligning the entries if any entry starts on the same line as the var/let and continues onto a later line. For example:

  // Previous behavior (with the over-indentation issue):
  let foo = someItem
        .getValue(), // Column-alignment (relative to `foo`) looks ok here...
      bar = otherItem
        .getValue()
  
  getAThing()
    .andDoStuffWithIt()
  let foo = someItem
        .getValue() // but looks over-indented with a single entry.
  getOtherThing()
    .andDoStuffWithIt()
  
  // New behavior after this fix:
  getAThing()
    .andDoStuffWithIt()
  let foo = someItem
    .getValue() // No column alignment in this case (good)...
  doOtherThing()
  
  let foo = someItem
    .getValue(), // or in this case (unfortunate, but far less common)...
    bar = otherItem
      .getValue()
  
  let foo = someItem.getValue(),
      bar = otherItem.getValue() // but still column-aligned in this case (good).

  Using this rule, as opposed to handling single and multi-entry PBDs differently, ensures that the as-typed-out indentation (where it's unclear how many entries the user intends to write) matches the selected-and-reindented indentation for multi-entry pattern binding decls.

• Scope of issue: The over-indentation affects all multi-line var/let declarations in Swift source files, and I expect many users would consider it a regression relative to the 5.2 behavior.

• Origination: Since the indentation implementation was overhauled in [SourceKit/CodeFormat] Re-work and improve the indentation implementation #30187

• Risk: Low. This only affects sourcekitd (not the compiler), and specifically only its CodeFormat request. It is a targeted fix that only affects the indentation within pattern binding declarations (var/let declarations).

• Testing: Updated and added regression tests for this change, regression tests pass, and I manually tested the changes in Xcode.

• Reviewer: @benlangmuir (on the master branch PR)

Resolves rdar://problem/63309288

[nathawes]

@swift-ci please test