Debugging Memory Leaks page references non-existent sanitizer flag

[ADKaster]

Describe the bug

The overview section mentions that on linux, there is a sanitizer=leak option to swiftc. This option does not exist, as LeakSanitizer is a component of AddressSanitizer. The section below valgrind on AddressSanitizer mentions how to ensure LSAN is enabled on linux.

To Reproduce

Steps to reproduce the behavior:

https://www.swift.org/documentation/server/guides/memory-leaks-and-usage.html#tools-and-techniques

On Linux: Swift has built-in LeakSanitizer support that can be enabled using the -sanitize=leak compiler flag.

Expected behavior

Document the LSAN integration with sanitize=address as explained more fully later in the document

Actual behavior

Misleading documentation overview

[ADKaster]

Closed by #959