[Parse] Improve diagnostics for consecutive identifiers

[rintaro]

Improve and simplify diagnostics for consecutive identifiers.

We should diagnose this right after the first identifier parsing.
For type decls and enum 'case', in parseIdentifierDeclName().
For variable/constant names, in parsePattern() .

Previously, it used to emit superfluous diagnostics.
For instance:

let ABC DEF = 12

test.swift:2:1: error: expected expression

^
test.swift:1:5: error: type annotation missing in pattern
let ABC DEF = 12
    ^

class Foo Bar : Equatable { }

test.swift:1:15: error: expected '{' in class
class Foo Bar : Equatable { }
              ^
test.swift:1:27: error: statement cannot begin with a closure expression
class Foo Bar : Equatable { }
                          ^
test.swift:1:27: note: explicitly discard the result of the closure by assigning to '_'
class Foo Bar : Equatable { }
                          ^
                          _ = 
test.swift:1:27: error: braced block of statements is an unused closure
class Foo Bar : Equatable { }
                          ^
test.swift:1:27: error: expression resolves to an unused function
class Foo Bar : Equatable { }
                          ^~~

[rintaro]

@swift-ci Please smoke test