AsmPrinter: Stop creating DebugLocs

While looking at a heap profile of a clang LTO bootstrap with -g, I
noticed that 2.2% of memory in an `llvm-lto` of clang is from calling
`DebugLoc::get()` in `collectVariableInfo()` (accounting for ~40% of
memory used for `MDLocation`s).

I suspect this was introduced by r226736, whose goal was to prevent
uniquing of `DebugLoc`s (goal achieved, if so).

There's no reason we need a `DebugLoc` here at all -- it was just being
used for (in)convenient API -- so the fix is to pass the scope and
inlined-at directly to `LexicalScopes::findInlinedScope()`.

llvm-svn: 229459