Make swift::SourceLoc and swift::SourceRange usable as a key type for DenseSet/DenseMap.

[mhong]

One use case is using DenseSet to manage a set of source locations, so that we
emit at most one compiler diagnostic per location (see
#18402).

[mhong]

@lattner Who would you suggest we loop in for review? Thanks.

[lattner]

I'd suggest changing this to:

SMLoc::getFromPointer(DenseMapInfo<const char *>::getEmptyKey())))

for consistency.

[lattner]

I can review it, LGTM.

[jrose-apple]

I do think you should use SourceLoc for your original test case, though. No reason to store twice as many pointers.

[mhong]

@jrose-apple, I tried out a simple example like:

public func test() {
  var a = 1.0
  a += 2.0
}

In the SIL code, I find that the insts that read the old a in a += 2.0 and the insts that wrte to the new a in a += 2.0 could share the same start loc, but have a different end loc.

e.g. I saw these two insts:

%4 = begin_access [modify] [static] %0 : $*Double

1: inst->getLoc().getSourceRange() = {Start = {Value = {
      Ptr = 0x877d061 "a += 2.0\n\n  let x = Tensor<Float>(1.0)\n}\n"}}, End = {Value = {
      Ptr = 0x877d061 "a += 2.0\n\n  let x = Tensor<Float>(1.0)\n}\n"}}}

vs

%5 = struct_element_addr %4 : $*Double, #Double._value

1: inst->getLoc().getSourceRange() = {Start = {Value = {
      Ptr = 0x877d061 "a += 2.0\n\n  let x = Tensor<Float>(1.0)\n}\n"}}, End = {Value = {
      Ptr = 0x877d066 "2.0\n\n  let x = Tensor<Float>(1.0)\n}\n"}}}

The former reads the old a, and the latter starts the computation for doing +=.

So it'd be safer to store the loc pair. We don't expect to generate a lot of sends/recvs warnings, but if performance ever becomes a concern, we can revisit this (e.g. maybe just warn on the first K occurrences globally, or within each function.)

[mhong]

@swift-ci please test

[jrose-apple]

I mean, even the range isn't unique per instruction, because of implicit expressions, or just AST-level expressions that take more than one instruction. You could also have unrelated diagnostics at the same location.

[mhong]

@jrose-apple Based on the feedback, I added DenseMapInfo specialization for SourceLoc as well. Once we downstream this patch into tensorflow branch, I can change the callsite to use SourceLoc as the key.

Please let me know if there are more comments. Thank you!

[mhong]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 0a09541

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 0a09541

[jrose-apple]

This seems reasonable to me!

[mhong]

Thank you @jrose-apple! I'll attempt a merge.

[mhong]

@swift-ci please test and merge

[mhong]

@swift-ci please test and merge