fix CoreCLR build errors with clang5.0. (#15477)

fix CoreCLR build errors with clang5.0. (#15477)

* spmi: delete the explicit call to the constructor.

MethodContext was already initialized through `new MethodContext();`

* add an artificial using of lambda param.

the define `#define PAL_EXCEPT(dispositionExpression)`
can declare expression that has zero or one param. If it has zero param, then `__param` in the lambda is unused, prevent clang complains by adding this change.