[ThinLTO] Always parse module level inline asm with At&t dialect (PR46503)

[ThinLTO] Always parse module level inline asm with At&t dialect (PR46503)

clang-cl passes -x86-asm-syntax=intel to the cc1 invocation so that
assembly listings produced by the /FA flag are printed in Intel dialect.
That flag however should not affect the *parsing* of inline assembly in
the program. (See r322652)

When compiling normally, AsmPrinter::emitInlineAsm is used for
assembling and defaults to At&t dialect. However, when compiling for
ThinLTO, the code which parses module level inline asm to find symbols
for the symbol table was failing to set the dialect. This patch fixes
that. (See the bug for more details.)

Differential revision: https://reviews.llvm.org/D82862