rustc_codegen_ssa should write temporary files to the output directory

[ChrisDenton]

In most places in the compiler temporary files are written to the output directory (using tempdir_in). E.g.:

rust/compiler/rustc_metadata/src/fs.rs

Lines 48 to 50 in 15c4cce

	let metadata_tmpdir = TempFileBuilder::new()
	.prefix("rmeta")
	.tempdir_in(out_filename.parent().unwrap_or_else(|| Path::new("")))

rust/compiler/rustc_codegen_ssa/src/back/archive.rs

Lines 504 to 506 in 15c4cce

	let archive_tmpdir = TempFileBuilder::new()
	.suffix(".temp-archive")
	.tempdir_in(output.parent().unwrap_or_else(|| Path::new("")))

The one exception is compiler/rustc_codegen_ssa/src/back/link.rs which writes to the OS temp directory (note the use of tempdir instead of tempdir_in):

rust/compiler/rustc_codegen_ssa/src/back/link.rs

Lines 103 to 105 in 15c4cce

	let tmpdir = TempFileBuilder::new()
	.prefix("rustc")
	.tempdir()

I don't think there is a good reason to maintain this inconsistency?

Note that this affects -C save-temps where you have to go hunting for the relevant temp file (or redirect the platform-specific temp directory environment variable).

[jieyouxu]

Related: #138475