[Parser] Reset C++ lexer just past the whole ASTGen-parsed syntax node

Instead of "spinning" the C++ lexer, consuming tokens uptil we get past
the point where ASTGen told us the end of the syntax node was, just
reset the lexer to exactly that position. This is more efficient, but
also fixes problems where we would end up skipping past a `>` that had
been split.

Author: DougGregor

include/swift/Parse/Parser.h

@@ -1361,10 +1361,11 @@ class Parser {
       return nullptr;
     }
 
-    // Spin the lexer until we get to the ending location.
-    while (Tok.getLoc().getOpaquePointerValue() < endLocPtr &&
-           !Tok.is(tok::eof))
-      consumeToken();
+    // Reset the lexer to the ending location.
+    StringRef contents =
+        SourceMgr.extractText(SourceMgr.getRangeForBuffer(L->getBufferID()));
+    L->resetToOffset((const char *)endLocPtr - contents.data());
+    L->lex(Tok);
 
     return makeParserResult(astNode);
   }

test/ASTGen/types.swift

@@ -35,3 +35,7 @@ func test12a(i: Int) {
 func test13(body: (_ value: Int) -> Void, i: Int) {
   body(i)
 }
+
+func test14() {
+  _ = Array<Array<Array<Int>>>().count
+}