[CodeCompletion] Eat identifier characters after CC position

Text on editors (e.g. Xcode) may contain identifiers following to CC
position which should be considered as "filtering text" for the code
completion.

For example, a user types '@A' the CC position is between '@' and 'a'.
The user probably expects attributes starting with 'a'.

Eat identifier characters after CC token in Lexer. By this change,
for instance for '@<token>IB class', the parser now can detect this
is a start of class declaration and emit class attributes completions.

rdar://problem/46103294
(cherry picked from commit 56ce861)

Author: rintaro

lib/Parse/Lexer.cpp

@@ -2386,6 +2386,8 @@ void Lexer::lexImpl() {
   case 0:
     switch (getNulCharacterKind(CurPtr - 1)) {
     case NulCharacterKind::CodeCompletion:
+      while (advanceIfValidContinuationOfIdentifier(CurPtr, BufferEnd))
+        ;
       return formToken(tok::code_complete, TokStart);
 
     case NulCharacterKind::BufferEnd:

test/IDE/complete_decl_attribute.swift

@@ -2,6 +2,7 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=AVAILABILITY2 | %FileCheck %s -check-prefix=AVAILABILITY2
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD2 | %FileCheck %s -check-prefix=KEYWORD2
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD3 | %FileCheck %s -check-prefix=KEYWORD3
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD3_2 | %FileCheck %s -check-prefix=KEYWORD3
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD4 | %FileCheck %s -check-prefix=KEYWORD4
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD5 | %FileCheck %s -check-prefix=KEYWORD5
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD_LAST | %FileCheck %s -check-prefix=KEYWORD_LAST
@@ -63,6 +64,10 @@ class C {}
 // KEYWORD3-NEXT:             Keyword/None:                       usableFromInline[#Class Attribute#]; name=usableFromInline
 // KEYWORD3-NEXT:             End completions
 
+@#^KEYWORD3_2^#IB
+class C2 {}
+// Same as KEYWORD3.
+
 @#^KEYWORD4^#
 enum E {}
 // KEYWORD4:                  Begin completions, 5 items