Linker: Add flag to override linkage rules

Add a flag to lib/Linker (and `llvm-link`) to override linkage rules.
When set, the functions in the source module *always* replace those in
the destination module.

The `llvm-link` option is `-override=abc.ll`.  All the "regular" modules
are loaded and linked first, followed by the `-override` modules.  This
is useful for debugging workflows where some subset of the module (e.g.,
a single function) is extracted into a separate file where it's
optimized differently, before being merged back in.

Patch by Luqman Aden!

llvm-svn: 235473

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/test/Linker/Inputs/override-with-internal-linkage-2.ll b/llvm/test/Linker/Inputs/override-with-internal-linkage-2.ll
new file mode 100644
index 00000000000..2f54a1511fa
--- /dev/null
+++ b/llvm/test/Linker/Inputs/override-with-internal-linkage-2.ll
@@ -0,0 +1,4 @@
+define internal i32 @foo(i32 %i) {
+entry:
+  ret i32 4
+}