Various `ForInStmtSyntax` builder regressions

[stevapple]

Description

We've found a series of regressions when building ForInStmtSyntax, including:

• awaitKeyword is defaulted to .keyword(.await) instead of nil, which is unexpected for most users;
• BasicFormat fails to add spacing after awaitKeyword.

Steps to Reproduce

let buildable = ForInStmtSyntax(pattern: PatternSyntax("element"), sequenceExpr: ExprSyntax("array")) {
  ExprSyntax("print(element)")
}
print(buildable.formatted())

Outputs:

for awaitelement in array {
    print(element)
}

Expected:

for element in array {
    print(element)
}

[ahoppen]

rdar://107921603