Fix init-captures for generic lambdas.

For an init capture, process the initialization expression
right away.  For lambda init-captures such as the following:
const int x = 10;
 auto L = [i = x+1](int a) {
   return [j = x+2,
          &k = x](char b) { };
 };
keep in mind that each lambda init-capture has to have:
 - its initialization expression executed in the context
   of the enclosing/parent decl-context.
 - but the variable itself has to be 'injected' into the
   decl-context of its lambda's call-operator (which has
   not yet been created).
Each init-expression is a full-expression that has to get
Sema-analyzed (for capturing etc.) before its lambda's
call-operator's decl-context, scope & scopeinfo are pushed on their
respective stacks.  Thus if any variable is odr-used in the init-capture
it will correctly get captured in the enclosing lambda, if one exists.
The init-variables above are created later once the lambdascope and
call-operators decl-context is pushed onto its respective stack.

Since the lambda init-capture's initializer expression occurs in the
context of the enclosing function or lambda, therefore we can not wait
till a lambda scope has been pushed on before deciding whether the
variable needs to be captured.  We also need to process all
lvalue-to-rvalue conversions and discarded-value conversions,
so that we can avoid capturing certain constant variables.
For e.g.,
 void test() {
  const int x = 10;
  auto L = [&z = x](char a) { <-- don't capture by the current lambda
    return [y = x](int i) { <-- don't capture by enclosing lambda
         return y;
    }
  };
If x was not const, the second use would require 'L' to capture, and
that would be an error.
Make sure TranformLambdaExpr is also aware of this.

Patch approved by Richard (Thanks!!) 
http://llvm-reviews.chandlerc.com/D2092

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196454 91177308-0d34-0410-b5e6-96231b3b80d8