Remove uses of the redundant ".reset(nullptr)" of unique_ptr, in favor of ".reset()"

It's also possible to just write "= nullptr", but there's some question
of whether that's as readable, so I leave it up to authors to pick which
they prefer for now. If we want to discuss standardizing on one or the
other, we can do that at some point in the future.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213439 91177308-0d34-0410-b5e6-96231b3b80d8
clang/test/Misc/backend-optimization-failure.cpp: Appease to add -triple=x86_64.

FIXME: Could this be made generic?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213441 91177308-0d34-0410-b5e6-96231b3b80d8
PR20356: Fix all Sema warnings with mismatched ext_/warn_ versus
ExtWarn/Warnings. Mostly the name of the warning was changed to match the
semantics, but in the PR20356 cases, the warning was about valid code, so the
diagnostic was changed from ExtWarn to Warning instead.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213443 91177308-0d34-0410-b5e6-96231b3b80d8
Use the dereferenceable attribute on C99 array parameters with static

In C99, an array parameter declarator might have the form:
  direct-declarator '[' 'static' type-qual-list[opt] assign-expr ']'

where the static keyword indicates that the caller will always provide a
pointer to the beginning of an array with at least the number of elements
specified by the assignment expression. For constant sizes, we can use the
new dereferenceable attribute to pass this information to the optimizer. For
VLAs, we don't know the size, but (for addrspace(0)) do know that the pointer
must be nonnull (and so we can use the nonnull attribute).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213444 91177308-0d34-0410-b5e6-96231b3b80d8