c++: Update mangling of lambdas in expressions

https://github.com/itanium-cxx-abi/cxx-abi/pull/85

 clarifies that
mangling a lambda expression should use 'L' rather than "tl".

gcc/cp/ChangeLog:

	* mangle.cc (write_expression): Update mangling for lambdas.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-generic-mangle1.C: Update mangling.
	* g++.dg/cpp2a/lambda-generic-mangle1a.C: Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>