[Assembler] Improve diagnostics for inline assembly.

Summary:
Keep a vector of LocInfos around; one for each call to EmitInlineAsm.
Since each call to EmitInlineAsm creates a new buffer in the inline asm
SourceMgr, we can use the buffer number to map to the right LocInfo.

Reviewers: rengolin, grosbach, rnk, echristo

Reviewed By: rnk

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D29769

llvm-svn: 294947