llvm-mc --assemble prints an initial `.text` from `initSections`.
This is weird for quick assembly tasks that do not specify `.text`.
Omit the .text by moving section directive printing from `changeSection`
to `switchSection`. switchSectionNoPrint now correctly calls the
`changeSection` hook (needed by MachO).
The initial directives of clang -S are now reordered. On ELF targets, we
get `.file "a.c"; .text` instead of `.text; .file "a.c"`.
If there is no function, `.text` will be omitted.
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