Pass down an optimization level to the Clang linker invocation

[artemcm]

Resolves rdar://105261863

[artemcm]

cc @dmcyk.

[artemcm]

@swift-ci test

[dcci]

Does this allow arbitrary -Oxxx ? (e.g. -O, -O1, -Oz)?
Looks like it, but I want to double check.

[artemcm]

Does this allow arbitrary -Oxxx ? (e.g. -O, -O1, -Oz)?
Looks like it, but I want to double check.

It doesn't, because Swift (driver or frontend) does not have such flags.
Swift only has:

-O                   - Compile with optimizations
-Osize               - Compile with optimizations and target small code size
-Ounchecked          - Compile with optimizations and remove runtime safety checks
-Oplayground         - Compile with optimizations appropriate for a playground

So the mapping that myself and @dmcyk came up with is:

-O and -Ounchecked -> -O3
-Onone             -> -O0
-Osize             -> -Os
-Oplayground       -> -O0 (that's what it maps to in swift-frontend) 

[dcci]

sounds good.

[artemcm]

Users can override this with a more fine-grained Clang flag if they wish to, adding to their swiftc command:

-Xclang-linker -O2

[dmcyk]

Left two minor comments, but otherwise LGTM!

[artemcm]

@swift-ci test

[artemcm]

@swift-ci test Windows platform