The core.runtime module always overrides the default parameter value for
constructor calls. MaxAlignment is not required because a class can be
created on the stack with the `scope' keyword.
libphobos/ChangeLog:
* libdruntime/core/runtime.d (runModuleUnitTests): Use scope to new
LibBacktrace on the stack.
* libdruntime/gcc/backtrace.d (FIRSTFRAME): Remove.
(LibBacktrace.MaxAlignment): Remove.
(LibBacktrace.this): Remove default initialization of firstFrame.
(UnwindBacktrace.this): Likewise.
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