test: ensure that we do not regress the standard library isolation

Add a test to ensure that the standard library does not accidentally end
up with symbols in the `llvm::` namespace.  The LLVMSupport symbols for
now are namespaced to `__swift::__runtime` with the desire that they
will move into `swift::` and become vended in the swift namespace.

Author: compnerd

test/lit.cfg

@@ -290,6 +290,7 @@ config.filecheck = inferSwiftBinary('FileCheck')
 config.llvm_dwarfdump = inferSwiftBinary('llvm-dwarfdump')
 config.llvm_readelf = inferSwiftBinary('llvm-readelf')
 config.llvm_dis = inferSwiftBinary('llvm-dis')
+config.llvm_nm = inferSwiftBinary('llvm-nm')
 config.sourcekitd_test = inferSwiftBinary('sourcekitd-test')
 config.complete_test = inferSwiftBinary('complete-test')
 config.swift_api_digester = inferSwiftBinary('swift-api-digester')
@@ -437,6 +438,7 @@ config.substitutions.append( ('%swift-llvm-opt', config.swift_llvm_opt) )
 config.substitutions.append( ('%llvm-dwarfdump', config.llvm_dwarfdump) )
 config.substitutions.append( ('%llvm-readelf', config.llvm_readelf) )
 config.substitutions.append( ('%llvm-dis', config.llvm_dis) )
+config.substitutions.append( ('%llvm-nm', config.llvm_nm) )
 config.substitutions.append( ('%swift-demangle-yamldump', config.swift_demangle_yamldump) )
 config.substitutions.append( ('%Benchmark_O', config.benchmark_o) )
 config.substitutions.append( ('%Benchmark_Driver', config.benchmark_driver) )

test/stdlib/llvm-support-odr-violation.test-sh

@@ -0,0 +1,2 @@
+// RUN: %llvm-nm --defined-only -C %platform-module-dir/%target-library-name(swiftCore) | %FileCheck %s
+// CHECK-NOT: [^:]llvm::