Visit lambda capture inits from RecursiveASTVisitor::TraverseLambdaCapture().

Summary:
rL277342 made RecursiveASTVisitor visit lambda capture initialization
expressions (these are the Exprs in LambdaExpr::capture_inits()).

jdennett identified two issues with rL277342 (see comments there for details):

- It visits initialization expressions for implicit lambda captures, even if
  shouldVisitImplicitCode() returns false.

- It visits initialization expressions for init captures twice (because these
  were already traveresed in TraverseLambdaCapture() before rL277342)

This patch fixes these issues and moves the code for traversing initialization
expressions into TraverseLambdaCapture().

This patch also makes two changes required for the tests:

- It adds Lang_CXX14 to the Language enum in TestVisitor.

- It adds a parameter to ExpectedLocationVisitor::ExpectMatch() that specifies
  the number of times a match is expected to be seen.

Reviewers: klimek, jdennett, alexfh

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23204

llvm-svn: 278933