[libcxx] Improve code generation for vector::clear().

Summary:
By manipulating a local variable in the loop, when the loop can
be optimized away (due to no non-trivial destructors), this lets
it be fully optimized away and we modify the __end_ separately.

This results in a substantial improvement in the generated code.

Prior to this change, this would be generated (on x86_64):

    movq    (%rdi), %rdx
    movq    8(%rdi), %rcx
    cmpq    %rdx, %rcx
    je    LBB2_2
    leaq    -12(%rcx), %rax
    subq    %rdx, %rax
    movabsq    $-6148914691236517205, %rdx ## imm = 0xAAAAAAAAAAAAAAAB
    mulq    %rdx
    shrq    $3, %rdx
    notq    %rdx
    leaq    (%rdx,%rdx,2), %rax
    leaq    (%rcx,%rax,4), %rax
    movq    %rax, 8(%rdi)

And after:

    movq    (%rdi), %rax
    movq    %rax, 8(%rdi)

This brings this in line with what other implementations do.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25241

llvm-svn: 298601