Compare file:line prefix case-insensitively on win32
Paths are case insensitive on windows and rustc and compiletest may disagree on casing.
Fixes test compile-fail/circular_modules_main
auto merge of #8003 : crnobog/rust/case-insensitive-error-prefix, r=cmr

Paths are case insensitive on windows and rustc and compiletest may disagree on casing.
Fixes test compile-fail/circular_modules_main on win32